Between 4.5 and 4.6, many structures in M_, options_ and elsewhere which used to be character vectors are now cell arrays.

We need to compile the list of all those structures, and document that change in the release notes.

Also, we need to provide a compatibility layer and/or some tips on how to adapt existing code.

The function convert_oo.m should also be updated, and moved alongside convert_dyn_45_to_44.m to the new subfolder for compatibility layers.