fix: protect members of StateTransitionGraph

[redeboer]

#460 decorated the StateTransitionGraph with attr.s and set its members to frozen. However, edge_props and node_props are dicts, meaning that you can still modify their values (and even their keys by using pop etc.). So, if we want the StateTransitionGraph to be truly frozen, their attributes can only be exposed if they are frozen themselves as well. See also #470.

Note: some additional checks have been added on the constructor of the STG.

[codecov]

Codecov Report

Merging #479 (dc62014) into master (eb70bd7) will decrease coverage by 0.03%.
The diff coverage is 77.41%.

@@            Coverage Diff             @@
##           master     #479      +/-   ##
==========================================
- Coverage   90.15%   90.11%   -0.04%     
==========================================
  Files          22       22              
  Lines        3270     3279       +9     
  Branches      828      827       -1     
==========================================
+ Hits         2948     2955       +7     
- Misses        166      168       +2     
  Partials      156      156              

Flag	Coverage Δ
unittests	90.11% <77.41%> (-0.04%)	⬇️

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Impacted Files	Coverage Δ
src/expertsystem/reaction/topology.py	90.96% <77.41%> (-0.41%)	⬇️