Enable grouping sets in Mondrian 3.X for MonetDB database

[llorieb]

I'm using MonetDB with Pentaho 8.1 to analyze a very large fact table (around 200 millons rows and more than 50 columns). MonetDB Jun2020 release implemented grouping sets, and I think this feature would increase aggregates performance noticeably.

Today, using grouping sets in Pentaho Mondrian 3.X is a feature limited to a couple of databases (Oracle, DB2 and Greenplum), and it'd be great having MonetDB in such a selected group.