CSL/CTMC: TIme bounded until, zero time bounds.

[ivan.zapreev]

In test

test/functional_tests/ctmc/csl/operators/time_bounded_until/csl_bounded_until_01

for

P{<0.1}[ tt U[0,0] a4 ]

we get

   Result: ( 1.0e+00, 1.0e+00, 1.0e+00, 1.0e+00 )

But it should be

   Result: ( 0.0e+00, 0.0e+00, 0.0e+00, 1.0e+00 )

[ivan.zapreev]

In case the time bound is set to [0,0] we expect it to be an unbounded until.