add cell_offsets attribute to Domain, use it in Region

- simplify Domain.get_cell_offsets()
move fem/region.py -> common/region.py
new Domain.get_evaluate_cache()
clean up whitespace
remove mesh argument from constructors, remove all mesh references

- update Probe.probe() to use Domain.get_evaluate_cache()
- simplify Probe.__init__()
move discrete/fem/probes.py -> discrete/probes.py
update examples for moved and updated probes