Issues computing connectivity with CTF system

I got a report that the connectivity computation was problematic. If you have a CTF-270 system, the function find_ch_connectivity gives you the connectivity of the CTF-275 system. Code to reproduce:

import mne
from mne.channels import find_ch_connectivity

contrast = mne.evoked.read_evokeds('./S1_discrimination_conds-ave.fif',
                                   condition='contrast_RP', verbose=None)

connectivity, ch_names = find_ch_connectivity(contrast.info, ch_type='mag')
assert connectivity.shape[-1] == len(contrast.info['ch_names'])

here is the file: https://www.dropbox.com/s/bae8rjdu068ws14/S1_discrimination_conds-ave.fif?dl=0.

My suggestion would be to make _compute_ch_connectivity public and deprecate the find_ch_connectivity function. The guesswork in the function introduces more problems than solves.

Reproduce the report with the linked FIF file and inspect find_ch_connectivity and _compute_ch_connectivity in mne/channels/channels.py. Compare the computed connectivity and channel count for the CTF-270 data, then determine whether exposing the internal function and deprecating find_ch_connectivity is appropriate. Done means the CTF-270 computation no longer uses the wrong system assumptions and the reported shape remains correct.