test: add angular distribution tests from pycompwa

[spflueger]

First test is ported from pycompwa, together with some fixture code.

I also made a small change to the variable names, as they were not consistent.

The second one I will tackle tomorrow. But this will serve as the baseline for all other distribution test.
@redeboer @Leongrim @sebastianJaeger , take a look if you like the test setup. Ideally I would like you to possibly extend this with your channels.

Closes #181

TODOs:

• add second pycompwa distribution check
• add scipy as a test dependency

[codecov]

Codecov Report

Merging #182 (376011f) into master (cd560fa) will increase coverage by 0.80%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   82.85%   83.66%   +0.80%     
==========================================
  Files          13       13              
  Lines         700      704       +4     
  Branches       96       97       +1     
==========================================
+ Hits          580      589       +9     
+ Misses         87       83       -4     
+ Partials       33       32       -1     

Flag	Coverage Δ
unittests	83.66% <100.00%> (+0.80%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ensorwaves/physics/helicity_formalism/amplitude.py	75.74% <100.00%> (+2.16%)	⬆️
...nsorwaves/physics/helicity_formalism/kinematics.py	85.41% <100.00%> (+0.10%)	⬆️

[spflueger]

I would just keep this PR as it is and @Leongrim and @sebastianJaeger can add their channels in follow up PRs. It also reflects the title this way.

[redeboer]

Good job! Just have some comments about the structure of the angular_distributions folder. That's related to #182 (comment) as well

[redeboer]

So apart from #182 (comment), I think we can move on with this PR