SliceJacobiPC should allow variable length slices

[JHopeCollins]

The first implementation of SliceJacobiPC requires that all slices are the same length, and the boundaries coincide with the Ensemble slices.

I think relaxing the requirement for constant length slices should be pretty straightforward and could be useful to try.

Relaxing the requirement for the Slice/slice boundaries to coincide would be much messier and less useful, we usually run with one or two timesteps per slice anyway.