SliceJacobiPC should allow variable length slices #195
Description
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.