Specifically, allocate storage for 'masstrans' and 'gravtrans' terms
per well completion (perforation), as well as compressible
transmissibilities (scalar per completion). Calculate compressible
quantities by treating each completion as an interface.
Introduce a new structure, 'completion_data', to collect static and
dynamic discretisation data pertaining to each completion (e.g.,
productivity indices, gravity potentials and density ratio
operators). Pass this structure, rather than individual fields, into
affected CFS_TPFA entry points.
Compile tested only.
