For OPM data, it would be convenient to allow mne coreg to adjust more than just the head_mri_t (by writing -trans.fif files, which it does currently).

Even with our existing OPM dataset from years ago, the head and MRI coordinate frames are taken as equal, which they shouldn't be. We should allow for using the current (in the GUI) sensor<->MRI relationship and use the current LPA/Nasion/RPA (from the MRI) to correct the LPA/Nasion/RPA in info['dig'], which in turn should update the head coordinate frame definition, which in turn should change the rest of info['dig'], info['chs][ii]['loc'], info['dev_head_t'], etc.

Adapted from #11405