Fixed issue in computation of cumulative rewards

[tquatmann]

There was a problem with computing R=?[C<=t]-like properties on CTMCs. With the rather simple 2-state model

ctmc

module main
x : [0..1] init 0;
[] x=0 -> 6 : (x'=1);
endmodule

rewards
	x=0: 1;
endreward

we currently get

Model checking property "1": R=? [C<=1/10] ...
Result (for initial states): inf
Time for model checking: 0.000s.

This PR fixes the issue and adds the model as a test case.

The problem was that we are supposed to scale the values obtained before the left truncation point.
However, the case where the left truncation point is 0 was handled incorrectly: Essentially the scaling had been applied twice: once when multiplying with foxGlynnResult.weights.front() in what is now line 705 and then again with foxGlynnResult.totalWeight in line 737.

I also improved numerical stability in the weight adjustment. The previous implementation sometimes resulted in negative weights because the weights were summed up in a non-optimal order.

[sjunges]

LGTM