Harmonize __repr__ and __str__ for all classes in base
Fix Dat.__repr__ argument order
Fix Global.__repr__ argument order
Add unit tests for pyop2 API repr and str
CUDA and OpenCL check for Maps to be populated on construction
Recreating a Kernel from its repr isn't safe due to preprocessing
Merge pull request #185 from OP2/repr

Harmonize __repr__ and __str__ and add unit tests for them

* Fixes argument order errors in the Dat and Global repr
* Harmonizes __repr__ and __str__ for all classes in base
* Adds unit tests for pyop2 API repr and str, where possible
  using the repr to recreate an object