[WIP] Nonlinear

[martin-springer]

• Code changes are covered by tests
• Code changes have been evaluated for compatibility/integration with TrendAnalysis
• New functions added to __init__.py
• API.rst is up to date, along with other sphinx docs pages
• Example notebooks are rerun and differences in results scrutinized
• Updated changelog

[codecov-commenter]

Codecov Report

❌ Patch coverage is 89.28571% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.21%. Comparing base (b5eb6e2) to head (16ac19f).

Files with missing lines	Patch %	Lines
rdtools/degradation.py	84.55%	21 Missing ⚠️
rdtools/plotting.py	95.31%	3 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #517      +/-   ##
===============================================
- Coverage        96.79%   96.21%   -0.58%     
===============================================
  Files               12       12              
  Lines             2342     2566     +224     
===============================================
+ Hits              2267     2469     +202     
- Misses              75       97      +22     

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