This commit introduces a new public method, activeRegions(), that
retrieves those region IDs that contain at least one active cell.
We furthermore extend the cells() method to support lookup of
arbitrary region IDs. Non-active region IDs produce empty cell
ranges.
Intended use case is
for (const auto& reg : rmap.activeRegions()) {
const auto& c = rmap.cells(reg);
// use c
}