Add ECG rate classification tutorial

[victoriajrosenthal]

This PR adds an ECG python notebook intended for rate classification. The code loops through specified number of records, identifies r-peaks, and calculates heart rate and short-term HRV. Classifies heart rates into three categories: brachycardia, tachycardia, and sinus rhythms. Provides summary statistics for each classification.

[briangow]

Thanks @victoriajrosenthal . I added some "Suggested" and "Optional" comments in the code.

Additionally, I'd suggest removing the !pip install block at the beginning for consistency with other notebooks in this repo (which don't do the environment setup), and the empty block at the end. Optionally, you could also add an overview at the top describing what the notebook does and more comments about what a given block is doing.

Happy to merge this as-is but if you have time to make some of these changes that'd be great.

[briangow]

Optional: you define rec_path but then don't use it in pn_dir. You could so something similar to what you do further below when you pass {folder_path} to pn_dir.

[briangow]

Suggested: remove this feedback comment.

[briangow]

Optional: Same thought as above, you define rec_path but then don't use it.

[briangow]

Suggested: I'd remove this block since it isn't doing anything.

[briangow]

Optional: You could add a blank line at the end as this is standard practice.