Projective cont.

[ziyi-zhang]

This PR keeps small changes to projective integrator related stuff.

Changed:

• Turn the flag sqrt_scale_mass into a float in [0, 1] to transform sample contribution with x'=x^(1-alpha). This gives more flexibility to control the suppression of outliers.

Question:

• Why octree_scatter_inc is by default false?

(re-gen docstring before merging)