feat: add caching of constant sub expressions in SympyModels

[spflueger]

Closes #100

[spflueger]

I'm keeping it like this for now. Not the best user interface, but I suspect we have more issues regarding the Model interface and the gradient as well. So I think this Model interface just does not cut it...

[codecov]

Codecov Report

Merging #247 (737df5d) into master (8485e82) will increase coverage by 1.46%.
The diff coverage is 80.90%.

@@            Coverage Diff             @@
##           master     #247      +/-   ##
==========================================
+ Coverage   79.71%   81.17%   +1.46%     
==========================================
  Files          13       13              
  Lines         641      712      +71     
  Branches       97      119      +22     
==========================================
+ Hits          511      578      +67     
- Misses         93       96       +3     
- Partials       37       38       +1     

Flag	Coverage Δ
unittests	81.17% <80.90%> (+1.46%)	⬆️

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

Impacted Files	Coverage Δ
src/tensorwaves/estimator.py	80.00% <72.72%> (-5.37%)	⬇️
src/tensorwaves/model.py	77.33% <81.81%> (+17.09%)	⬆️

[redeboer]

Good job!
I left a few comments. It's a bit obscure why the macos job keeps failing now, looking into it. This is a problem that sporadically happened before, so this has to be tackled anyway. (Still, it's strange that this PR makes the macos job more unstable.)

[redeboer]

A series of PRs related to the callbacks is coming up that might fix the CI problem seen here.