Improve docs by giving multidimensional example for GROMACS

[davidlmobley]

For those less familiar with pandas it would probably be helpful to give an example of GROMACS analysis where two lambda values are changed simultaneously; right now the docs only give an example with a single lambda variable, which makes the syntax less complex. For two dimensions I ended up with something like mbar.delta_f_.loc[[(0.0, 0.0)], [(1.0, 1.0)]] but figuring out the right placement of brackets and parens was tricky to me (I'm still a pandas newb).

[orbeckst]

@dotsdl if you manage to give some thoughts to docs then this might be one of the examples to address.

[orbeckst]

@davidlmobley do we have an example for this case in alchemtest?

[davidlmobley]

Not in alchemtest yet that I'm aware of. I'm checking in the group for data and will revisit after that. If you don't hear back please harass me.

[harlor]

Btw. If you don't want to use the lambda values as index explicitly you can also use mbar.delta_f_.values[0, -1].