Make analyze more flexible

[pabloferz]

We should give the user the ability to switch free energy integration routines. We can provide this as an argument to analyze.

CC @ricalessandri

[ricalessandri]

As part of this, we should take care of fixing something in the functions that generate the free energy of FUNN and ABF. In fact, they show some systematic error (left), that gets fixed if the forces are integrated using this py script from the SSAGES repo (right):

[ricalessandri]

This is how the integration error seem to look in 1D CV case (butane w/ HOOMD-blue and FUNN):

[pabloferz]

#249 should help at least with the errors you are seeing here. The more flexible interface for analyze still needs some tinkering.