In test_abstract_operator.py   - corrected identation which led to generation of redundant tests   - implemented unit test for FourthRankIsotropicTensor.to_memoryview.
In test_abstract_operator.py
  - corrected identation which led to generation of redundant tests
  - implemented unit test for FourthRankIsotropicTensor.to_memoryview.

sbrisard/janus,sbrisard/janus,sbrisard/janus