Blender Git Loki

Completely refactored sph fluid particles. Only the very core of the algorithm remains
the same, but big changes have happened both on the outside and on the inside.

New UI:
* The old parameters were quite true to the underlying algorithm, but were quite obscure
from a users point of view. Now there are only a few intuitive basic parameters that
define the basic fluid behavior.
** By default particle size is now used to determine the interaction radius, rest
density and spring rest lengths so that it's easy to get stable simulations by simply
emitting particles for a few frames and adjusting the particle size (easy when the
particle size is drawn) so that the fluid appears continuous (particles are touching
eachother).
** Stiffness - in reality most fluids are very incompressible, but this is a very hard
problem to solve with particle based fluid simulation so some compromises have to be
made. So the bigger the stiffness parameter is the less the fluid will compress under
stress, but the more substeps are needed for stable simulation.
** Viscosity - how much internal friction there is in the fluid. Large viscosities also
smooth out instabilities, so less viscous fluids again need more substeps to remain
stable.
** Buoancy - with high buoancy low pressure areas inside the fluid start to rise against
gravity, and high pressure areas start to come down.

* In addition to these basic parameters there are separate advanced parameters that can
either be tweaked relative to the basic parameters (or particle size) or defined
independently.
** Repulsion - the stiffness parameter tries to keep the fluid density constant, but this
can lead to small clumps of particles, so the repulsion keeps the particles better
separated.
** Stiff viscosity - the normal viscosity only applies when particles are moving closer to
eachother to allow free flowing fluids. Stiff viscosity also applies smoothing to
particles that are moving away from eachother.
** Interaction radius - by default this is 4 * particle size.
** Rest density - by default this is a density that the particles have when they're packed
densely next to eachother.
** Spring rest length - by default this is 2 * particle size.

* There are also new options for 3d view particle coloring in the display panel to show
particle velocity and acceleration. These make it easier to see what's happening in the
fluid simulations, but can of course be used with other particles as well.

* Viscoelastic springs have some new options too. The plasticity can now be set to much
higher values for instant deletion of springs as the elastic limit is exeeded. In addition
to that there is an option to only create springs for a certain number of frames when a
particle is born. These options give new possibilities for breaking viscoelastic fluids.

New in the code:
* Most of the fluids code is now thread safe, so when particle dynamics go threaded there
will be a nice speed boost to fluids as well.
* Fluids now use a bvh-tree instead of a kd-tree for the neighbor lookups. The bvh-tree
implementation makes the code quite a bit cleaner and should also give a slight speed
boost to the simulation too.
* Previously only force fields were calculated with the different integration methods, but
now the fluid calculations are also done using the selected integration method, so there
are again more choices in effecting simulation accuracy and stability. This change also
included a nice cleanup of the whole particle integration code.

As the internals are pretty stirred up old particle fluid simulations will probably not
work correctly straight away, but with some tweaking the same level of control is still
available by not using the "relative versions" of the advanced parameters (by default these
are not used when loading old files).