Make pickling to work on domains/problems objects (Fix #79)

[marcofavorito]

Proposed changes

Fixes a bug in the wrapper _getstate of the __getstate__ method (262b347f95586a361f1da23e89381dd83aa3191a), since the cached hash value __hash was not set if the hash was never computed on an object; and in that case, the pop from the dict-state was not available.

This PR also add tests that make sure the pickling works.

Fixes

Fix #79.

Types of changes

What types of changes does your code introduce? Put an x in the boxes that apply

• [X] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• [ ] I have read the CONTRIBUTING doc
• [ ] Lint and unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works

Further comments

n/a

[codecov-commenter]

Codecov Report

Merging #80 (db9b14d) into main (e9669dd) will increase coverage by 0.01%. The diff coverage is 100.00%.

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@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
+ Coverage   89.66%   89.67%   +0.01%     
==========================================
  Files          22       22              
  Lines        1180     1182       +2     
  Branches      192      193       +1     
==========================================
+ Hits         1058     1060       +2     
  Misses         92       92              
  Partials       30       30              

Flag	Coverage Δ
unittests	89.67% <100.00%> (+0.01%)	:arrow_up:

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Impacted Files	Coverage Δ
pddl/helpers/cache_hash.py	94.59% <100.00%> (+0.30%)	:arrow_up: