PotentialParticle minor changes (documentation, unnecessary prerequisites and contact between fixed bodies)

After discussions with @vsangelidakis and local experience:

• a bit of documentation clarification
• removing an unnecessary metis "prerequisite" for PP or PB
• 0fefc20b which is the biggest change since fixed PotentialParticle-shaped bodies can now interact (for real). There was a short-circuit before to save computational time, if I understood correctly, but with our pre-existing Body.mask functionality and PotentialParticle.isBoundary (which also has its own short-circuit), it was redundant (and would prevent one to test PP-related contact laws in a displacement-controlled loading of an interaction). It's obviously not fully backwards-compatible in terms of behavior but I would say it is an improvement.