Fixed bugs in Grounder::GuaranteeConsistency.

The most important bug regards Reground() after GuaranteeConsistency():
Regrounded ungrounded relevant terms may very well affect which clauses
are relevant.
The new procedure is to reground independent of relevance first, and in
a second step to remove the irrelevant clauses. This removal is done by
throwing away the shallow copy and creating a new one.

Other bugs concerned the considered plies for Ply::lhs_rhs,
Ply::relevant, etc.