Document how to use Gromacs

[MarionBWeinzierl]

Talk to James Gebbie about how to use Gromacs and add the information to the docs

[ptheywood]

I've added some basic Gromacs documentation in PR #67.

This is also a duplicate of #37.

[ptheywood]

Now #67 is merged, the very basic Gromacs documentation can be exapended by editing software/applications/gromacs.rst

[ptheywood]

Closing this as the docs taskforce is being closed. The docs page can still be expanded if required with bede-speicifc additions if required.