BUG, MAINT: Remaining dig issues

[larsoner]

In the long term we should:

• [x] Remove _read_dig_points, it should make use of standardized functions (#8622)
• [ ] Make read_dig_polhemus_isotrak actually read channel names where possible (they are embedded in the file at least sometimes)
• [ ] Make .elp reading by read_custom_montage triage automatically between isotrak and BESA (currently just assumes BESA)

Here is a sample for (3), renamed .log instead of .elp to make GitHub accept it:

channel_locations.log