split numkit into separate package

[orbeckst]

For modularity and maintainability we should move numkit into a separate small package.

• [x] remove numkit from GromacsWrapper
• [ ] add a stub that explains where to get the package (once the import fails)
• [x] make separate package that installs with
  • setup.py
  • pypi
  • conda TODO
• [x] is tested separately
• [x] has its own docs

[orbeckst]

Note that this will require existing code such as MDPOW to explicitly install numkit. – @ianmkenney