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>

Iterating, using conditionals, and then sorting the results seems less explicit about what's going on, and more convoluted than just directly accessing the dict itself in the desired order, but I can live with it since it achieves the same thing functionally if you prefer it @adykstra.