BUG: Correct evaluate function for ddt0 in CrankNicolson scheme. Fixes

The function evaluate was returning true every outer loop, triggering
the re-calculation of ddt0 in every outer loop.

The evaluation of the term ddt0 should be performed once per time step.
The corrected function updates the timeIndex of ddt0 to avoid the
re-evaluation of this term in the outer loops.