Support Julia function being called as a pari potential

[vchuravy]

@dallasfoster asked for this on Slack.

Two avenues that I see immediatly

1. Mimic pair_python
   • Benefits:
   • Usable outside LAMMPS.jl
   • Downsides:
   • Requires linking LAMMPS against libjulia
2. Enhance the LAMMPS API to make it possible to register a custom pair after loading the libraries
   • Benefits:
   • pair_python seems to only support one Python potential at a time? We could potential support many Julia potentials
   • Potential for better integration with LAMMPS.jl
   • Maybe lower overheads
   • Downsides:
   • Users need to use LAMMPS.jl
   • Unproven path

From a cursory research for #2 it seems that we would need to expose a custom Pair class that takes raw function pointers and forwards the method implementations and then register that with PairCreatorMap which is a map from string to PairCreator which is a typedef Pair *(*PairCreator)(LAMMPS *).

cc: @rohskopf

[vchuravy]

28 makes quite a bit of progress on this.