Refactor _trace and _force into a class
Remove debugging code, to squash
Remove leftovers, to squash
Remove debugging code, to squash
Turn -Werrror back on
Run laziness tests for CUDA and OpenCL
Use op2.base._force.evaluate(...) systematically to force par_loop evaluation
Fix lazy evaluation dependencies specifications

Fix Mat::zero_rows is a RW operation
Fix cuda.Mat.zero missing specification