This commit includes the fraction of pore-volume whose CNV targets
are violated as a new per-iteration quantity in the INFOITER file
(--output-extra-convergence-info=iteration), with the column header
"CnvErrPvFrac". We collect the values which are already calculated
in
BlackoilModel<>::getReservoirConvergence()
and store these as a pair of numerator and denominator in the
ConvergenceReport class. Note that we need both the numerator and
the denominator in order to aggregate contributions from multiple
ranks.
While here, also make a few more objects 'const' and calculate
column widths directly instead of the maximum number of characters
in writeConvergenceHeader().
