Performance testing of degeneracy checking for PT generation

[ewenlawrence]

This pull request adds a performance benchmark for comparing generating process tensors with and without degeneracy checking, using a S_z coupling operator of varying spin size.

Pull Request Check List

• The contribution has been discussed and agreed on in the Issue section.
• Code contributions do its best to follow the zen of python.
• The automated test are all positive:
  • tox -e py36 (to run pytest) the code tests.
  • tox -e style (to run pylint) the code style tests.
  • tox -e docs (to run sphinx) generate the documentation.
• Added test for changed/added code.
• API code contributions include input checks (defensive code).
• API code contributions include helpful error messages.
• The documentation has been updated:
  • docstring for all new functions/methods/classes/modules.
  • consistent style of all docstrings.
  • for new modules: /docs/pages/modules.rst has been updated.
  • for api contributions: /docs/pages/api.rst has been updated.
  • for api contributions: tutorials and examples have been updated.

[piperfw]

Hi @ewenlawrence, sorry hadn't noticed this sitting here. Thanks for putting your tests into a convenient format for us.

The generated plot doesn't look like the one currently in that paper, is the one in the paper a separate one you or Gerald have produced?

[gefux]

Great, thank you! This closes #115