kim-api "ignored" particles

[ellio167]

Make changes to support an "ignored" particle status that a model will essentially ignore when computing energies and forces, etc. This will make it possible to use KIM PMs in a "hybrid-mode" where they can be combined to build up the desired interactions. This will require backward-incompatible changes.

• change particleContributing to contain an new extensible enumeration with three values: contributing, padding, and ignored. (This will lead to a lot of operator=() calls. We should profile the impact that this has and if it is significant consider solutions. Perhaps, move implementation to headers so inlining can be done?)

• add support for compute arguments that are of type: extensible enumeration for particle class (or whatever we call it). Will need to design a good solution for this (use opaque pointers? support explicit specific extensible enumerations as needed? Probably the latter.)

• change name of particleContributing argument to something more appropriate.

• New definition of Neighbor Lists: as in 2.0, with the following new required behavior if there are ignored particles.

  • ignored particles have zero neighbors
  • ignored particles do not show up as neighbors of any other particles