Add CPM (Kassinopoulos & Mitsis 2020)

[tsalo]

Closes #23.

This is currently just a draft with a direct, untested translation from MATLAB to Python.

Proposed Changes

• Add functions related to calculating the cardiac pulsatility model (CPM) metric.

Change Type

• [ ] bugfix (+0.0.1)
• [X] minor (+0.1.0)
• [ ] major (+1.0.0)
• [ ] refactoring (no version update)
• [ ] test (no version update)
• [ ] infrastructure (no version update)
• [ ] documentation (no version update)
• [ ] other

Checklist before review

• [ ] I added everything I wanted to add to this PR.
• [ ] [Code or tests only] I wrote/updated the necessary docstrings.
• [ ] [Code or tests only] I ran and passed tests locally.
• [ ] [Documentation only] I built the docs locally.
• [X] My contribution is harmonious with the rest of the code: I'm not introducing repetitions.
• [ ] My code respects the adopted style, especially linting conventions.
• [X] The title of this PR is explanatory on its own, enough to be understood as part of a changelog.
• [ ] I added or indicated the right labels.
• [ ] I added information regarding the timeline of completion for this PR.
• [X] Please, comment on my PR while it's a draft and give me feedback on the development!

[tsalo]

The code successfully runs on random arrays! I still need to make sure that my variable names and comments reflect the actual variables, hopefully with input from the original creator.

[rgbayrak]

I have had issues translating code from MATLAB to python. I would check the output signals side-by-side before merging this. More specifically there is a discrepancy in signal filtering.