Fix segfault in subtraction

This is a quick hack fix to make DP3 subtraction functional again.

I notice that inside DDECal, the variable itsRequestedSolInt is used for the nr of timesteps in the current interval, but during the last interval this is not correct, because the nr of timesteps might not by divisable by the requested solint. I've also renamed itsSolInt to itsRequestedSolInt, because IMHO it was confusing, also since there are various other its(N)SolInt(Count) variables that have very similar names.

This seems to have been introduced in bf495ff8