refactor: wrap fit result in class

[redeboer]

Wrapped the optimize dict output in a FitResult class. It's a rather simple attr data container that wraps what was previously in a dict form. Optimizer-specific info has been put under a specifics attribute.

[codecov]

Codecov Report

Merging #254 (5303b84) into master (3c788a0) will decrease coverage by 1.23%.
The diff coverage is 61.53%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
- Coverage   81.17%   79.94%   -1.24%     
==========================================
  Files          13       13              
  Lines         712      743      +31     
  Branches      119      124       +5     
==========================================
+ Hits          578      594      +16     
- Misses         96      111      +15     
  Partials       38       38              

Flag	Coverage Δ
unittests	79.94% <61.53%> (-1.24%)	⬇️

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

Impacted Files	Coverage Δ
src/tensorwaves/interfaces.py	82.14% <55.88%> (-17.86%)	⬇️
src/tensorwaves/optimizer/minuit.py	93.22% <100.00%> (ø)
src/tensorwaves/optimizer/scipy.py	93.10% <100.00%> (ø)

[redeboer]

One thing that's still missing is the covariance matrix. We have to think about a data type for that though.

[redeboer]

So the way I see this PR now is that it only (1) replaces the dict fit result with a class and (2) offers missing additional info (particularly the covariance matrix or hessian). This means that #165 is not closed.

Btw, the fact that the dict has been replaced with a class, helped fix one typo:

tensorwaves/src/tensorwaves/optimizer/scipy.py

Line 145 in b2bb1f0

"interations": res.nit,