USD import: fixed crash getting shader value.

Getting a value from an invalid shader input was causing
a crash in the UMM conversion invocation. I added validty
checks for the input attribute in several places to
avoid this. Also, minor formatting fix.

USD export: Skel Root validation.

Added function for verifying that skinned prims
and skeletons are properly grouped under a common
SkelRoot. Also added a Fix Skel Root export
option to attempt to fix the hierarchy if the Skel
Root is invalid.

USD export: avoid creating redundant root prim.

If a root prim path is set in the params, now checking
if a root object matching the root path name already
exists in the Blender scene. Clearing the root prim
path in the params and printing a warning if it does.
This is to avoid prepending the root prim path redundantly.

USD IO: material conversion improvements.

Updated the USD Preview Surface texture node
import code to handle UDIM tiles that don't
start a 1001. Performed miscellaneous cleanup
to make code more robust.

Fixed logic for the MDL material fallback behavior
to import the USD Preview Surface shaders only if
the material has no MDL shaders. I.e., it will not
load preview surface as a fallback if an MDL exists
but failed to load for some reason. This is much
more useful for debugging failures and also gives
the user an opportunity to fix a partially successful
MDL import. This refactor also fixes a significant
bug where the fallback would be used even if the MDL
import succeeds. Refactored the report_notification()
utility function in the UMM conversion code to return
more meaningful results.

Added logic to generate file names for packed texture
assets when exporting USD Preview Surface shaders.
Previously, such asset paths were left empty and were
omitted from the export.

Updated the UsdUVTexture shader conversion code to
handle the case where the file input has a connected
source, which may happen if this input is overridden
by an input on the parent material.

Made the logic for determining the color
space for texture assets when collecting UMM
source data more robust by handling the case
where a connected source input has no color
space specified. The fix is to also query
the shader's input attribute for this data.

Merge branch 'master' into universal-scene-description

USD import options description edit.

Shortened excessively long option descriptions
by removing information that should be included
in the documentation instead.

USD IO format fixes.

USD Preview Surface import as a fallback.

Added logic to fall back on importing existing
USD Preview Surface shaders if importing MDL
is selected as an option but the material has
no MDL shaders.

USD Export: Armature export improvements.

Now including the root prim in the skinned mesh
skeleton relationship path. Also, added logic to
avoid nesting SkelRoot prims in the USD, as such
nesting causes skeleton binding to fail as well
as crashes in Create.

Now iterating over the deform groups of the
evaluated mesh when setting joint weights
and indices, to ensure the vertex group
data is valid.

USD export: fixed linux an mac compile error.

USD IO: attribute conversion improvements.

Initial implementation of logic to import USD
attibutes as Blender custom properites, with options
to import all custom attributes or only those
attibutes in the 'userProperties' namespace.

New export option to add custom properties to the
'userProperties' USD attribute namespace. This
option is enabled by default.

Removed hidden functionality where custom properties named
with the prefix 'USD_' were being saved to properties on the
USD prim that have the same name, without the prefix. This
code was not type safe and could lead to unexpected behavior
in case of accidental property name collisions.

Added support for converting between USD int, float and
double vectors and Blender array type custom properties.

USD IO: enable presets.

Enable Operator Presets drop down menu
for the USD import/export operators.

USD IO: handle UMM Python module load error.

Clearing the Python import module error if loading
the UMM module failed. If we don't do this, the
Python unit test for USD will fail if the UMM
addon isn't installed. Also printing the Python
error in this case, if printing warnings is enabled.

USD export: fix build errors with latest master.

Merge branch 'master' of git.blender.org:blender into universal-scene-description

USD IO improved feedback.

Now parsing the Python notification dictionary object
returned by UMM to output warnings and error to the
Blender log.

Removing unneeded log message when setting the
default prim, as it can obscure more important
warnings and errors in the status bar.

USD IO options improvements

Added default values for the Default Prim Path,
Root Prim Path and Material Prim Path export
options. Now validating that these options are
set to well formed USD paths and raising an
error if these paths are invalid. This helps avoid
potential crashes when attempting to define
USD prims with invalid paths. Also updated import
shaders option menu tooltip.

USD Import: minor format fix

Merge remote-tracking branch 'blender_org/master' into temp-usd-latest-master

Merge remote-tracking branch 'blender_org/master' into temp-usd-latest-master