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>

I would not document it as end users are not supposed to create their own forward

or at least not expose it in mne namespace

my 2c