add citation.cff file

[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

All modified and coverable lines are covered by tests ✅

Project coverage is 96.10%. Comparing base (b3927da) to head (6c8e04c).

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #434      +/-   ##
===============================================
- Coverage        96.19%   96.10%   -0.10%     
===============================================
  Files               11       11              
  Lines             2209     2209              
===============================================
- Hits              2125     2123       -2     
- Misses              84       86       +2     

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