1. Edited _move_events to take into account metapopulation selection.
2. Replace bound_t.ndim with bound_t.shape.rank for TF Graph Mode running.
3. Revised index calculation in `_abscumdiff`.  Re-wrote test to accommodate.
4. Code rationalisation in FilteredEventTimesProposal to return a simpler data structure.
5. Changed dimension of results.extra field to trace `x_star`.
6. Edited tests in `tests.test_mcmc.py` for new `_abscumdiff` interface.