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 the function is pretty good now but I was wondering if there exists are more efficient implementation. I was playing around with using a hash value of the string and summing them up for all aspects. Obviously, that might lead to collisions but it was 10x the performance. If the profile still shows the function on the top, we might consider making a native group by for facts to push the functionality down. Just thinking out loud here...