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>

Names aren't a safe way to find this. I'd just do `len(pick_types(forward.info, meg=True)` or so, since it will rely on more tested/robust underlying code.