Made it possible to provide UserConstraints in Protege via a file
<ontolog>.db_prefs. Did it as simple as possible, no listener, no
saving. No tests.
Refactoring the SesameVirtualRepo constructors
Changed the SesameVirtualRepo and QuestDBVirtualStore such that the call
to setupRepository is not done in the constructors, but moved to a
method initialize. Such can a setter method be used to give the implicit
db constraints. Changed name of UserConstraints to ImplicitDBConstraints