Add Hilbert RVT (Harrison et al. 2020)

[tsalo]

Closes #10.

This adds a new function (chest_belt.hilbert_rvt()) to calculate the Hilbert transform-based RVT metric from Harrison et al. (2020). This implementation is based on the manuscript, rather than any available code (couldn't find any).

Proposed Changes

• Add metrics.chest_belt.hilbert_rvt() to calculate the new metric.
• Add a number of functions to metrics.chest_belt.utils module in support of the new 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.
• [X] [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.
• [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.
• [ ] 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!