Harmonize "landmarks" parameter across write_anat() and update_anat_landmarks()

[hoechenberger]

Currently, the parameters don't accept the same input data in both functions, see https://github.com/mne-tools/mne-bids/pull/977#issuecomment-1060007085

While looking into this, I also started thinking that get_anat_landmarks() should probably be rename to something like trans_to_mri_landmarks(), because the current name can be confusing.

Leaving this here to tackle in 0.11.