Implement RESV control mode for producers

This commit introduces support for the RESV control mode of
prediction (WCONPROD) and history-matching (WCONHIST) alike.  The
implementation uses class SurfaceToReservoirVoidage<> to compute
coefficients that convert component rates at surface conditions
(i.e., the primary degrees of freedom in the well residual) to phase
rates at reservoir condition.  The resulting coefficients can be
entered directly into system matrix of the linearised residual.

Note: We abuse the "distr" mechanism of struct WellControls to store
the conversion coefficients.  This may require refactorisation and
clarification at a later stage.  In the meantime, it allows for
transparent assembly of well equations--irrespective of surface- or
reservoir (voidage) rates.

Note: We do not yet support injectors controlled by total reservoir
voidage rate--either in history-matching (WCONINJH) or
prediction-scenario capacity.
Aesthetics: Turn if-else into switch()

This commit replaces an if-else-based query of the primaryVariable_
with the equivalent switch() statement for uniformity with the rest
of the implementation.