Well rates of distributed wells might be stored on multiple processes
but should be summed only once. Hence only the owner does the
summation with this commit.
This commit creates a single implementation function for deriving
'RateVector' objects that go into the guiderate calculation. In
particular, we now use the same implementation function for both the
well and the group levels. While here, also expose the group level
derivation as a free function and reimplement the FractionCalculator
version in terms of this free function. Finally, remove the
previous attempt at such a free function taking only the group name.
This function no longer exists in isolation and is only accessible
through the FractionCalculator.
This is in preparation of reporting guiderate values to the output
layer (summary and restart files).
