fix: throw invalid L error correctly

[spflueger]

• Throw exception correctly when using helicity formalism
• Test if the exception is raised as expected

[codecov]

Codecov Report

Merging #188 (7add890) into master (8b69eb8) will increase coverage by 0.16%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   82.24%   82.41%   +0.16%     
==========================================
  Files          13       13              
  Lines         704      705       +1     
  Branches       97       98       +1     
==========================================
+ Hits          579      581       +2     
+ Misses         90       89       -1     
  Partials       35       35              

Flag	Coverage Δ
unittests	82.41% <0.00%> (+0.16%)	⬆️

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	75.83% <0.00%> (+0.46%)	⬆️

[spflueger]

We could add a small test, that makes sure this error is thrown when this case actually appears.

[spflueger]

Ok done. Unfortunately the design of the IntensityBuilder makes the testing quite difficult. Hence the rather abstruse test. Improving that design can be put on our Todo list.

[redeboer]

Nice test! Although the syntax can indeed be improved, hopefully through #162