Implementation of LJGauss and OPP pair potential under hpmc.

[shihkual]

Description

I would like to help implement the LJGauss and OPP pair potential under the hpmc module, equivalent to the implementation in md.

Proposed solution

LJ-Gauss

1. Write PairPotentialLJGauss.cc and PairPotentialLJGauss.h, using the implementation in hoomd.md.pair as reference.
2. Write lj_gauss.py.
3. Include the three scripts to CMakeLists.txt.
4. Write Unittest?

OPP

1. Write PairPotentialOPP.cc and PairPotentialOPP.h, using the implementation in hoomd.md.pair as reference.
2. Write opp.py.
3. Include the three scripts to CMakeLists.txt.
4. Write Unittest?

Additional context

No response