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 don't think so... it is less explicit about what's being done. Iterating usually indicates "do something for each item in this sequence", whereas what you're trying to achieve is individually sub-selecting things from a sequence. But that sequence already has a very nice individual sub-selection method (dict access).

If you want to ensure the fields actually exist, you'd just do `if self.get('info', None) is not None:`, but I *think* we're guaranteed these fields all exist. It simplifies the code quite a bit just to say directly `ch_names = self.info['ch_names']` instead of needing to embed that within a conditional within a loop.