Pr/add ace

[Roddie01]

Pull request for implementing ACE into OQuPy, as agreed in #152. I still need to add documentation, but I ran tests, and they were all positive. The main feature is discretising the spectral density; we may need to discuss this, as it requires a user-input spectral density to allow thermalisation. In addition, it allows the user to choose how to discretise the spectral density. Currently, it's only fully set up to work with Bosonic environments with the aim of extending it further later on.

• [ x] The contribution has been discussed and agreed on in the Issue section.
• [ x] Code contributions do its best to follow the zen of python.
• [ x] The automated test are all positive:
  • [x ] tox -e py36 (to run pytest) the code tests.
  • [ x] tox -e style (to run pylint) the code style tests.
• [ x] Added test for changed/added code.
• 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 new functionality: /docs/pages/api.rst has been updated.
  • for new functionality: tutorials and examples have been updated.

[piperfw]

Hi @Roddie01, exciting to see this! Thanks for flagging the spectral density question. I suggest once @gefux and I have had a chance to look over the code we might want to arrange a quick meeting to discuss this and anything else we might spot. Helpful to have the Testing_ACE_Implementation.py to allow us to run an example (before merge it should be moved into examples/).

I remember one of the first questions was regarding PT format. It looks like you were able to resolve that quite easily with a converter function? We can discuss whether it makes sense for this to live in OQuPy or as an export function in ACE.