Fix stupid demo

fix spelling
remove unused variable
use public method instead of manually zeroing varea Dat
Refactor _trace and _force into a class
Remove debugging code
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
Fix unit test to account for device backends