Add AssignmentCollection.has_exclusive_writes

An assumption of pystencils is that output stencil writes never overlap. This allows massive parallelization without race conditions or atomics.

When I use my autodiff transformations I use this condition to check whether the assumption still hold for the backward assignments.

I like the idea to check restrictions already on the assignment collection. However, it somewhat overlaps with checks done here and documented here. Besides checking for double writes, we have to check for example also for read after write.

closed

Right, this essentially does the same. Closing.