All features are in

Need some more work on details
Several things:

- get healing length from settings
- do H2-H2 only with non-bonded interactions
- also correct vcoul
Revert until it's confirmed
Conditional compilation of H2-H2 code
Merge branch 'master' into H2
Only call H2-H2 if there are H2 molecules
Simplify if
Merge branch 'master' into H2
Merge branch 'master' into H2
Minor - update TODO
Merge pull request #53 from TuckermanGroup/H2

Add H2-H2 molecular potential by Diep and Johnson