Add smoke tests for metrics and fix miscellaneous bugs

[tsalo]

Closes None.

This fixes a number of bugs in the chest_belt, cardiac, and retroicor functions, and adds smoke tests for each of the implemented metrics (minus retroicor).

Proposed Changes

• Drop lags from RV calculation. We'll do it later.
• Drop downsampling from metrics, where possible.
• Add smoke tests for metrics. These tests only check that data of the right type and shape are outputted by the functions when provided inputs of the right types and shapes.
• Fix cardiac_phase implementation. I took the code directly from https://github.com/Brainhack-Donostia/bhg2018/blob/master/physiological-denoising/physio_denoising.ipynb this time.

Change Type

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

Checklist before review

• [X] I added everything I wanted to add to this PR.
• [X] [Code or tests only] I wrote/updated the necessary docstrings.
• [X] [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.
• [X] 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.
• [X] I added or indicated the right labels.
• [ ] I added information regarding the timeline of completion for this PR.
• [ ] Please, comment on my PR while it's a draft and give me feedback on the development!