TEST: Add (sub)Tests, refactor a bit
TEST: Add (sub)Tests, refactor a bit

* Add test for deleting source file on decompress
* Refactor function for checking decompression output precision to @staticmethod
* Add test for `prec=None` use case on `h5_to_cube`
* Refactor to add subTests for CubeToH5_Delete test.
bskinn/h5cube