Add water particle free energy datasets

[harlor]

As suggested in https://github.com/alchemistry/alchemlyb/pull/62 I want to add 3 new free energy data sets:

• One without an energy column
• One with a potential energy column
• One with a total energy column

These data sets are useful to test the calculation of the reduced potential for gromacs data sets in alchemlyb. See https://github.com/alchemistry/alchemlyb/issues/59.

[orbeckst]

I approved a bit prematurely because the entries in gmx.rst are still missing. But just ping me when you have added it and I'll merge.

[harlor]

Ok - I added the information to the gmx.rst

[orbeckst]

Thank you!