Added facts:canonical-grouping-key#1 and #2

Usage:
for $fact in ...
group by $key := facts:canonical-grouping-key($fact, ($facts:CONCEPT, $facts:PERIOD))
return...

I think that a precomputed grouping key (at least with concept covered) is a brilliant idea!
Also, a hash code at runtime seems appealing. Depending on what the next profile will say, I'll try it out :-)