Pyemma compatibility

[giopina]

I've created a function (get_gvecs_pyemma) that computes g-vectors and return them in a format compatible with what required by
pyemma.coordinates.featurizer.add_custom_function()
I've got it in a branch on my fork.
I'm not sure whether this might be useful for everybody or is a bit out of scope at the moment.

(btw, it's not a big difference from the already existing dump_gvec_traj, I just removed seq from the return values and changed the shape of the gvecs array.