Integrate over integration-domains instead of cells in the lowered/back-end implementation (#688)

Fixes #683.

* Distinguish between per-cell matrix solver and per-'integration domain' state in the FVM back-end, adding new index for FVM back-ends mapping CVs to integration domain index.
* Move to one event queue per integration domain, avoiding extra logic for synchronisation of times across gap-junction connected cells. This requires a merging of event lanes in `mc_cell_group` when constructing the staged events in the multi-event queue.
* Add synapses to gap junction example.