Tentative solution to problem of RESV injectors in `ifs_tpfa.c`

This is a tentative solution to Issue #360 concerning the current impossibility of specifying injector wells constrained by total reservoir volume flow rate (`RESV`) when using the incompressible solver `ifs_tpfa.c` and, by extension, the wrapping class `IncompTpfa`.

The suggested solution is to restrict the consistency checking on phase distributions, introduced in commit b7d1634, to producing wells only.  Using this code I can no longer reproduce the symptoms of Issue #360, but further testing is likely needed.