Blender Git Loki

Avoid breakage when there are no custom properties to be written

Tweaks and adjustments and abstractions and generalisations and tests

Always flatten arrays of arrays

Remove debug prints

Generic int/float/double matrix support

Support for matrices of type double

Alembic export: write scalar custom properties

Write scalar custom properties as array properties to Alembic. This is
also what's done by Houdini and Maya exporters, so it seems to be the
standard way of doing things.

Seems to work ok'ish if I look at it via `usdcat`.

Add support for arrays of int/float/double

Alembic export: Use correct time sampling for custom properties