New fetchers module to download/cache all YASA sample data

[remrama]

This PR moves all of the YASA test and tutorial data to a Zenodo repository and accesses it from YASA via Pooch. It adds a new fetchers.py module to YASA. See #188 for details.

In the process of this PR, I ended up making a number of small changes to remove FutureWarnings from seaborn, pandas, and mne.

There is some room for future additions to the fetchers module to fetch other public datasets, more focused on analysis than testing.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 77.39%. Comparing base (4dcd72d) to head (0bb67d5).

Files with missing lines	Patch %	Lines
src/yasa/features.py	0.00%	1 Missing ⚠️
src/yasa/fetchers.py	95.23%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
+ Coverage   77.24%   77.39%   +0.14%     
==========================================
  Files          13       14       +1     
  Lines        2659     2681      +22     
  Branches      324      325       +1     
==========================================
+ Hits         2054     2075      +21     
  Misses        568      568              
- Partials       37       38       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[remrama]

Btw @raphaelvallat I am leaving this hanging because of a possible concern with the tests. I opened an issue on Pooch to get some feedback (see fatiando/pooch#456 for details). But I'm not that worried about it, so we can probably move forward next week if we don't hear back. I'll un-draft and assign you whenever it's ready.