Add docstrings to indicators

[MarcSkovMadsen]

The docstrings make it so much easier to use the widgets in an IDE like VS Code.

[codecov]

Codecov Report

Merging #3359 (4d85435) into master (4d4afb7) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 4d85435 differs from pull request most recent head 37b7693. Consider uploading reports for the commit 37b7693 to get more accurate results

@@           Coverage Diff           @@
##           master    #3359   +/-   ##
=======================================
  Coverage   83.27%   83.27%           
=======================================
  Files         196      196           
  Lines       26607    26607           
=======================================
  Hits        22156    22156           
  Misses       4451     4451           

Impacted Files	Coverage Δ
panel/widgets/indicators.py	92.48% <100.00%> (ø)

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[MarcSkovMadsen]

Thanks for wrapping the docstrings @philippjfr . Could you enlighten me why you do it? (I would like to know such that you don't have to do it).

I normally work with a max -line-length of 100 and I can see Panels flake8 rule is 165