fix: improve output of YAMLSummary

[redeboer]

Two fixes for the YAMLSummary:

• Scalar tensors are converted to built-in complex or float, so that they are rendered more nicely in the output of YAMLSummary.
• YAMLSummary now also dumps in on_optimize_end(), so that the load_latest_parameters indeed returns the latest parameters (see test_callbacks.py).

(Rebased on #251)

[codecov]

Codecov Report

Merging #252 (64e790a) into master (d960746) will decrease coverage by 0.45%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
- Coverage   77.70%   77.25%   -0.46%     
==========================================
  Files          13       13              
  Lines         619      633      +14     
  Branches       92       96       +4     
==========================================
+ Hits          481      489       +8     
- Misses        103      106       +3     
- Partials       35       38       +3     

Flag	Coverage Δ
unittests	77.25% <60.00%> (-0.46%)	⬇️

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Impacted Files	Coverage Δ
src/tensorwaves/optimizer/callbacks.py	63.79% <60.00%> (-0.59%)	⬇️