Refactoring autonomous processes events

[Ezpeon]

I started trying to refactor some of the code in PR #629

My goal was to cleanly separate the actions from the natural transitions as they are a bit different from a modeling perspective and this was not reflected in various representations.

This is not yet complete, but any suggestions about whether what I am doing makes sense is appreciated.

[codecov]

Codecov Report

Attention: Patch coverage is 88.49903% with 59 lines in your changes missing coverage. Please review.

Project coverage is 85.42%. Comparing base (6e3db94) to head (5ffee71).

Files with missing lines	Patch %	Lines
...d_planning/model/mixins/natural_transitions_set.py	73.07%	21 Missing ⚠️
unified_planning/model/natural_transition.py	86.08%	16 Missing ⚠️
unified_planning/model/transition.py	90.66%	14 Missing ⚠️
unified_planning/test/test_model.py	33.33%	4 Missing ⚠️
unified_planning/io/pddl_writer.py	93.75%	3 Missing ⚠️
unified_planning/model/action.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@                       Coverage Diff                       @@
##           autonomous_processes_events     #646      +/-   ##
===============================================================
+ Coverage                        85.38%   85.42%   +0.03%     
===============================================================
  Files                              204      207       +3     
  Lines                            27653    27801     +148     
===============================================================
+ Hits                             23611    23748     +137     
- Misses                            4042     4053      +11     

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