addon for grating stepping with a sin fit

[MarcusZuber]

Experiment addon that performs a sin-fit for the grating stepping.

[codecov]

Codecov Report

Base: 88.36% // Head: 88.37% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d03e556) compared to base (4f2027c).
Patch coverage: 93.02% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #513   +/-   ##
=======================================
  Coverage   88.36%   88.37%           
=======================================
  Files         120      120           
  Lines        8306     8347   +41     
=======================================
+ Hits         7340     7377   +37     
- Misses        966      970    +4     

Impacted Files	Coverage Δ
concert/experiments/addons.py	83.71% <88.46%> (+0.37%)	⬆️
...gration/test_grating_interferometry_experiments.py	95.13% <100.00%> (-0.14%)	⬇️

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[tfarago]

I fixed flake8 and showed you how I do parentheses organization now (it's taken from some code checker, forgot which). It leads to longer code but is way more readable and doesn't break the formatting due to new positions of line breaks when you add new parameters, which is very nice. What do you think?