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>

@Eric89GXL @adykstra in the info case it helps because we display every field but apply rules to similar content types. If we only want to show 3 or 5 things I would not use iteration and just type it directly