RSP could do with more hooks

Either hooks or wrapper functions from star_lib.

At the moment it would be helpful to be able to access init_def from rsp_def.f90 so we can change rsp_ variables on the fly. There may be more things to think about as well.

Start by reading rsp_def.f90, focusing on init_def and how rsp_ variables are currently defined or accessed. Clarify which hooks or wrapper functions are required, then define done as providing a documented way to change the relevant rsp_ variables at runtime without direct inaccessible access.

Written by the indexing model from the issue text.