This commit introduces new helper functions
int EclIO::combineSummaryNumbers(int, int)
tuple<int, int> EclIO::splitSummaryNumber(int)
that know about the relation
combined = n1 + 32768*(n2 + 10)
This relation is typically used to combine one-based region IDs for
inter-region flows into a single 'NUMS' entry in the summary file.