RESV: Ignore phase distribution for non-PRODUCERs

The WellsManager class handles INJECTORS by assigning a phase
distribution (W->ctrls[i]->distr) that coincides with the injected
fluid for the corresponding well (e.g., {1,0,0} for WATER injectors in
a three-phase WATER/OIL/GAS simulation).  This, however, meshes poorly
with the restriction that all phase components must be ONE in the case
of wells constrained by total reservoir volume flow targets (RESV)
that was introduced in commit b7d1634.

This change-set limits the restriction on phase distributions to
PRODUCERs only and is a tentative solution to GitHub PR #360.
assemble_well_contrib(): Fix misprint in comment.
Merge pull request #396 from bska/resv-injectors

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