Squirmer

A commonly-used simple model for active colloids is the squirmer model. It's basically a sphere with a slip boundary condition that induces dipolar flow. There are only two parameters, the self-propulsion velocity and the dipolar characteristic of the field.

This merge request adds a rigid body type to PE that inherits from the sphere and overrides velFromWF to get the correct boundary condition. I will add a physical test case next week.

All of this was originally implemented by @burkard on top of my custom particle implementation. I have now taken his work (specifically, the boundary handling and the physical tests) and ported it on top of PE. I have a contributor license agreement from him on file.