Statistical MC: Fix infinite sampling for lower bounded P[F]/P[U]

Only F/U operators with upper bound have a bounded path length and can
thus skip the maximal path length checks.

Test cases (will continue sampling beyond the default max path length,
appearing to hang):

prism prism-examples/dice/dice.pm -pf 'P=?[ F>2 s=0 ]' -sim
prism prism-examples/dice/dice.pm -pf 'P=?[ F>2 s=0 ]' -sim -ctmc

After this fix, the usual error message for unbounded operators is
generated.

prism/src/simulator/sampler/SamplerBoundedUntilCont.java

@@ -167,7 +167,7 @@ public class SamplerBoundedUntilCont extends SamplerBoolean
 	@Override
 	public boolean needsBoundedNumSteps()
 	{
-		// Always bounded (although we don't know the exact num steps, just the time bound)
-		return true;
+		// Bounded if there is a non-finite upper-bound (although we don't know the exact num steps, just the time bound)
+		return ub < Double.POSITIVE_INFINITY;
 	}
 }

prism/src/simulator/sampler/SamplerBoundedUntilDisc.java

@@ -137,7 +137,7 @@ public class SamplerBoundedUntilDisc extends SamplerBoolean
 	@Override
 	public boolean needsBoundedNumSteps()
 	{
-		// Always bounded
-		return true;
+		// Bounded if there is an upper bound
+		return haveUpperBound;
 	}
 }