WIP: add Forward class with repr

To be able to see important information regarding the gain matrix, i.e.:

    In[1]: fwd
    Out[1]: <Forward | MEG channels: 118 | EEG channels: 63 | Source space: Surface with 8171 vertices>

Ahh, I see what you mean -- the `info` use case is done to promote code re-use / no duplication. I agree it makes sense there, probably not here.