adds MAD function

[Trybnetic]

This PR introduces the calculation of the Mean Amplitude Deviation metric

• adds mad function
• adds tests for mad function

[codecov]

Codecov Report

Attention: Patch coverage is 28.57143% with 5 lines in your changes missing coverage. Please review.

Project coverage is 71.50%. Comparing base (44d494d) to head (79b555c).
Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
paat/features.py	28.57%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
- Coverage   72.02%   71.50%   -0.52%     
==========================================
  Files           8        8              
  Lines         579      586       +7     
==========================================
+ Hits          417      419       +2     
- Misses        162      167       +5     

Flag	Coverage Δ
unittests	71.50% <28.57%> (-0.52%)	⬇️

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