Add modality-agnostic and peak-related metrics

First set of functions to compute metrics for peak distance and peak amplitude.

Inits not yet updated. Tests still needed.

Proposed Changes

[ ] 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.
[ ] My contribution is harmonious with the rest of the code: I'm not introducing repetitions.
[ ] My code respects the adopted style, especially linting conventions.
[ ] 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.
[x] Please, comment on my PR while it's a draft and give me feedback on the development!