[MRG] Memory refactor

[pmyers16]

PR Description

Perform complete detection (fit) one channel at a time, parallelizing the entire process when desired. This allows us to control memory usage by n_jobs instead of being stuck with heavy memory load dependent on the recording's n_chs.

Closes: #33

Merge checklist

Maintainer, please confirm the following before merging:

• [ ] All comments resolved
• [ ] This is not your own PR
• [ ] All CIs are happy
• [ ] PR title starts with [MRG]
• [x] whats_new.rst is updated
• [ ] PR description includes phrase "closes <#issue-number>"

[adam2392]

Can you confirm that things run on the notebook dataset now for you?

[codecov-io]

Codecov Report

Merging #38 (2d2c25b) into master (10d17ff) will increase coverage by 0.11%. The diff coverage is 67.74%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   72.23%   72.34%   +0.11%     
==========================================
  Files          13       13              
  Lines        1167     1150      -17     
==========================================
- Hits          843      832      -11     
+ Misses        324      318       -6     

Impacted Files	Coverage Δ
mne_hfo/utils.py	51.63% <33.33%> (+0.03%)	:arrow_up:
mne_hfo/detect.py	48.24% <34.78%> (-10.85%)	:arrow_down:
mne_hfo/base.py	78.21% <96.87%> (+3.36%)	:arrow_up:
mne_hfo/compare.py	100.00% <100.00%> (ø)

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