Fixed: spheres must be updated when spheres are accessed.

Three or so commits ago, the spheres were updated after every
KnowledgeBase::AddClause etc.

Now we avoid this, because it avoids adding a new Ply in the Grounder
for every clause and should hence improve performance. Instead, the
spheres are only built when actually needed, namely in
KnowledgeBase::Entails().

Another case where they are needed are KnowlegeBase's n_spheres(),
sphere(), spheres().
In particular, meta-variables in the PDL should range over the
names in the spheres, i.e., over Solver::names() [this is actually
very questionable, for it contains plus-names, and will probably be
changed]. Hence we these methods need to update the spheres as well
when needed.