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>

The other reason iterating probably isn't great idea is that in python iterating over dict items does not have a guaranteed order, so more explicitly selecting them one-by-one 1) cuts down on the number of lines needed (no loop at all), 2) correspondingly decreases the amount of indentation used, and 3) guarantees a consistent order. And in this particular use case, if we're guaranteed that the fields actually exist, we again cut down on lines and indentation because we won't even need the `if self.get('info', None) is not None` checks.