Add montage parameter to BaseRaw

[massich]

This PR adds montage parameter in BaseRaw

[massich]

So far this PR does something really dummy. It takes info[dig] which has been populated by _read_raw_file with a Digitization object and re-assigns it to info['dig'].

But this already raises some questions:
1 - who should have the set_montage responsibility? IMHO it should exposed through raw object (from channels mixin as it is it's fine), but it should be done in meas_info. Right now is done in montage.

2 - how can I separate to get 'dig' from the info reading in here:

mne-python/mne/io/fiff/raw.py

Line 158 in 5d2f027

info, meas = read_meas_info(fid, tree, clean_bads=True)

cc: @agramfort, @larsoner

[agramfort]

not the route we decided to follow