expose interaction fitting

[csbrasnett]

would be nice to have the possibility of just giving ff_inter (or something else) a .dat file so it returns just parameters for that interaction, without worrying about the rest of them or a bunch of the gromacs stuff

[csbrasnett]

This is already exposed enough. I think if people want to do this I'll make a small example of how to use the InteractionFitter in their own script and put it in the docs