stencil failure with gtmc that works for numpy

[gridtools]

stencil failure with gtmc that works for numpy

[gronerl]

This stencil has a field on both the target and value side of the assignment. This is undefined behavior right now, and we need to take a decision at some point whether we want to not allow this (and print a meaningful error for all backends) or work around it by automatically adding temporaries. This decision depends on the outcome some of the ongoing discussions around the design of the top-level IRs.
Note that this is not specific to the "gtmc" backend but applies to all compiled backends.

[egparedes]

Related to #65

[havogt]

Illegal self-assignment.