API : pass info dict (gain_info) to depth computation (cleaner)

Conflicts:
	mne/forward.py
	mne/minimum_norm/inverse.py
API : rename force_fixed to fixed in make_inverse_operator

Conflicts:
	mne/minimum_norm/inverse.py
	mne/minimum_norm/tests/test_inverse.py
FIX : fix in _prepare_forward pick_info + cleanup