Specifically, rename the obtuse structure 'disc_data' to the more targeted
'densrat_util' and hoist the compressible terms 'ctrans' and 'P' into
the 'cfs_tpfa_impl' structure. Moreover, rename the remaining fields
into something that makes sense in (almost) isolation. Update
compute_densrat_update() and cfs_tpfa_construct() accordingly.
This is in preparation of adding compressible well terms.