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...

The reason is that $aligned-filter is not a fact object. There are lists of values for each aspect, not single values.
However, this is very likely to go away when we get rid of explicit caching.