adding groupByNodes() function, same as groupByNode() w/ several nodes

This make it possible to draw such targets:
groupByNodes(metrics.server*.database.*.*.*,'sum',1,4,5)
Whereas groupByNode can only group by one node.

Actualy, it was already possible to do that with groupByNode, but this required to use such ugly patterns:
aliasSub(groupByNode(aliasSub(aliasByNode(metrics.server*.database.*.*.*,1,4,5),'\\.','_'),0,'sum'),'_','.')