This commit hooks the new helper function restore_cumulative() up to
the gateway restart function
RestartIO::load()
by changing the return type from "RestartValue" to
std::pair<RestartValue, SummaryState>
with the pair's '.second' being restore_cumulative()'s return value.
Update callers including unit tests accordingly, specifically such
that the gateway function
EclipseIO::loadRestart()
internally resets its Summary object's cumulative quantities using
'load().second'. This is, strictly speaking, a violation of the
"const" qualifier on EclipseIO::loadRestart(), but the language
permits the usage because the 'impl' pointer in this case will be a
constant pointer to a mutable 'Impl' object.
9e97dde466