The initial use case is calculating the phase-filled pore-volume
weighted average of the fluid mass densities per PVT region. This
value goes into calculating depth-corrected per-cell phase pressure
values such as the BPPO and BPPG summary vectors.
This class manages a single linear array which separately tracks the
averages' numerators and denominators as running sums per region and
region set. We pick this data structure to simplify the cross-rank
reduction needed in MPI parallel runs. Client code is expected to
add individual per-cell and per-phase contributions using the
addCell() member function and then call the accumulateParallel()
member to affect the cross-rank reduction. The averages will then
be available through the fieldValue() and value() member functions.
As a further view towards the initial use case, we track two
different types of average per phase--one for the phase-filled
volume and one for the pore-volume filled volume. The latter is the
average we would get for the case of the phase saturation being one
throughout the region. This alternative value is the fallback
option for the case of the phase saturation being identically zero
throughout the region.
