First attempt at merging gated clock domains with parent clock
domain. Currently too enthusiastic - does them all, rather than just where
they merge.

src:
	* V3Order.cpp (OrderVisitor::domainMatch): Created.
	(OrderVisitor::processDomains): move gated domain into
	parent. More explanatory commenting.
	* Sentree.h (SenTreeFinder::getSenItem): Created.