DOC: how to write a penalty and a datafit

[mathurinm]

closes #31 (also adds a BaseDatafit class and shows how to write a custom datafit)

[codecov-commenter]

Codecov Report

Merging #55 (e3d8f63) into master (b8ce7d3) will increase coverage by 6.43%. The diff coverage is 64.30%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   56.19%   62.63%   +6.43%     
==========================================
  Files          12       11       -1     
  Lines        1098      669     -429     
  Branches      242      110     -132     
==========================================
- Hits          617      419     -198     
+ Misses        406      217     -189     
+ Partials       75       33      -42     

Impacted Files	Coverage Δ
andersoncd/tests/test_docstring_parameters.py	73.91% <ø> (-0.73%)	:arrow_down:
andersoncd/penalties.py	43.11% <43.11%> (ø)
andersoncd/datafits.py	57.14% <57.14%> (ø)
andersoncd/solver.py	59.72% <59.72%> (ø)
andersoncd/data/synthetic.py	72.41% <69.23%> (-18.50%)	:arrow_down:
andersoncd/__init__.py	100.00% <100.00%> (ø)
andersoncd/data/__init__.py	100.00% <100.00%> (ø)
andersoncd/estimators.py	100.00% <100.00%> (ø)
andersoncd/tests/test_estimators.py	100.00% <100.00%> (ø)
andersoncd/utils.py	28.76% <0.00%> (-4.11%)	:arrow_down:
... and 7 more

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[mathurinm]

https://292-306296806-gh.circle-artifacts.com/0/dev/add.html

[mathurinm]

Merge ?

[QB3]

I merge when green !