Support any elc file with set_montage

Describe the new feature or enhancement

As discussed with @rob-luke, we think it could be interesting to have the possibility to set a montage with any elc file with MNI coordinates.

Describe your proposed implementation

Given a custom elc montage file my-montage.elc with MNI coordinates standardised the same way as other existing elc files, it would be great to have an API like this:

# Detailed implementation
my_montage = mne.channels.make_standard_montage('path/to/my-montage.elc')
raw.set_montage(my_montage)

# With also the possibility to use it in one step
raw.set_montage('path/to/my-montage.elc')

Describe possible alternatives

Alternatives include having a separate function set_custom_elc_montage which takes the raw or epochs or evoked object and the montage path as arguments.

[rob-luke]

Great catch. Maybe worth adding to the new tutorial too?

[agramfort]

ok why not then. PR welcome