Added functions for finding alternates and groups of alternates to provglish.prov

Two functions added:

	prov.fetch_all_alternates(graph) -- Takes a graph, and returns all entities that have explicit
	alternates (including specializations). This function does not, at the moment, treat entities
	as alternates of themselves (though according to PROV-CONSTRAINTS this is implicitly true.)

	prov.fetch_all_alternate_groups(graph) -- Takes a graph, and returns a list of groups of entities.
	The entities in these groups are related to each other because they are alternatives or specializations
	of one another. This function obeys all the PROV-CONSTRAINTS, but does not return groups of one entity,
	unless that entity is explicitly described as being an alternate of itself.

Test fixture added:
	charlie.ttl

Test cases added:
	prov-test.Check_alternates contains two tests that test functionality of the functions added, using
	charlie.ttl.