Cleanup

remove unused argument
Fix, offset computation

no longer assume partitions' first index is a multiple of the partition size
move offset computation in compute_partition
Refactor base.ParLoop::compute

Orchestrate MPI operations in base.ParLoop instead of the backend subclasses
squash with previous

adapt sequential according to the previous commit refactoring
OpenMP: push colour iteration to python level
Plan: remove MPI specific properties
Plan: variable renaming
OpenMP: derive FakePlan from _GenericPlan
Make PPlan.echo __str__ method of _GenericPlan
We need a plan for any indirect loop

Conflicts:
	pyop2/openmp.py