Vollkornaffe

Once we've converged on style for the neo-hookean one, I'll update the corotated code to match.

This isn't the solution just yet: In `g2p2g_generic`, we read the value, pass it to `particle_g2p2g`, and write it back afterward. But that overrides changes from the `DruckerPragerPlasticity`.

Noticeable improvement in the 2D examples. The material doesn't seem to slide as on ice, see the screenshots below. Before:  After: 

Hmm, it does look like the fluid example is worse off as the number of escaping particles is increased. I'm not sure why that is. 

Is it too crazy to think that this offers another way to solve https://github.com/BrendanParmer/NodeToPython/issues/95?