Density map based SPH boundaries

[msiglreith]

We currently don't support boundary handle for our SPH based particle methods.
The common approach are particle based boundary but they require sampling of triangle meshes and have a few flaws.
Recently a new approach has been proposed based on density map grids, which seems to be more robust compared to the particle boundaries

Reference:
https://animation.rwth-aachen.de/media/papers/kb17.pdf