Fix issue #313 (different results with Nan's and Zeros in power series)

[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 96.21%. Comparing base (c59708a) to head (2338d92).

Additional details and impacted files

@@               Coverage Diff               @@
##           development     #442      +/-   ##
===============================================
+ Coverage        96.17%   96.21%   +0.03%     
===============================================
  Files               11       12       +1     
  Lines             2251     2270      +19     
===============================================
+ Hits              2165     2184      +19     
  Misses              86       86              

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[mdeceglie]

Looks good. I suggest we make a minor change to the docstrings to remind us and users about the x≥0 assumption

[martin-springer]

Looks good. I suggest we make a minor change to the docstrings to remind us and users about the x≥0 assumption

@mdeceglie - Yes, that sounds good to me. I committed your additions.