fix: add mass*width factor to relBW w/o ff

[redeboer]

Adds a factor of mass * width to the relativistic Breit-Wigner without form factor as well. Previously, this only happened for the rel. BW with form factor.

Note: this more or less restores the distribution of the example workflow to the first distribution shown in #187 (comment).

To summarize (the red line initial fit model can be ignored):

1. 574fe16 (v0.1.3, before with FF fix of fix!: use es.FormFactor in _RelativisticBreitWigner #187)
   

2. c35f270 (after fix!: use es.FormFactor in _RelativisticBreitWigner #187)
   

3. 1f7d1be (this PR)
   

[codecov]

Codecov Report

Merging #189 (1f7d1be) into master (e0b02d4) will decrease coverage by 0.23%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #189      +/-   ##
==========================================
- Coverage   82.41%   82.17%   -0.24%     
==========================================
  Files          13       13              
  Lines         705      707       +2     
  Branches       98       98              
==========================================
  Hits          581      581              
- Misses         89       91       +2     
  Partials       35       35              

Flag	Coverage Δ
unittests	82.17% <0.00%> (-0.24%)	⬇️

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	75.27% <0.00%> (-0.56%)	⬇️

[spflueger]

Looks good!