Specifically, introduce utilities compute_fpress() and
compute_flux(). The former is needed to implement the latter across
external boundary faces. Moreover, interface pressure values are
needed to evaluate fluid properties on faces (specifically R/B).
Add small gateway routine, cfs_tpfa_fpress(), to allow callers to
recover interface pressure values. Re-implement cfs_tpfa_press_flux()
in terms of compute_fpress() and compute_flux().
Also, add fields 'fpress' and 'fpaccum' to struct cfs_tpfa_impl.