feat: dataclass implementation

[APN-Pucky]

I quickly drafted a demo implementation of dataclasses instead of dicts. I considered

1. Backwards compatibility functions:

• __setitem__ and __getitem__ to mimic old dict-behavior like e.g. self["beamA"], but with a deprecation warning in order to get people to change their code.

2. Minimal modifications to the tests to maximize backwards compatibility:

• RuntimeError -> TypeError
• BREAKING CHANGE: Since the classes do not inherit from dict anymore we can not directly compare against a dict, however the dataclass.asdict can be used in the tests now.
• BREAKING CHANGE: LHEInit and LHEProcInfo previously could be instantiated without parameters, but I don't think that makes sense. Turning all their dataclass fields to optional also is ugly in my eyes.

3. Removal of fieldnames variable, but added again for backwards compatibility as a dynamic @property that reads the fields.

TODOs and considerations:

• one could add deprecation warnings to fieldnames() property and __setitem__/__getitem__.
• LHEVersion should not be a float (stupid AI...)
• => Test value of LHEVersion
• Add tests of the fieldnames() property to be the same as previously
• Coverage decrease, due to untested backwards-compatibility functions ... => corrected by adding backwards-compatibility tests

Closes: #303

[codecov]

Codecov Report

❌ Patch coverage is 98.36066% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.82%. Comparing base (55ec452) to head (7c9aea8).

Files with missing lines	Patch %	Lines
src/pylhe/__init__.py	98.36%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #307      +/-   ##
==========================================
+ Coverage   95.10%   95.82%   +0.72%     
==========================================
  Files           2        2              
  Lines         347      407      +60     
  Branches       63       59       -4     
==========================================
+ Hits          330      390      +60     
  Misses         10       10              
  Partials        7        7              

Flag	Coverage Δ
unittests-3.10	95.82% <98.36%> (+0.72%)	⬆️
unittests-3.11	95.82% <98.36%> (+0.72%)	⬆️
unittests-3.12	95.82% <98.36%> (+0.72%)	⬆️
unittests-3.13	95.82% <98.36%> (+0.72%)	⬆️
unittests-3.9	95.82% <98.36%> (+0.72%)	⬆️

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[Copilot]

Pull Request Overview

This PR converts the LHE data classes from dict-based implementations to proper dataclasses for improved type safety and clarity. The implementation maintains backwards compatibility through dictionary-like access methods and properties.

• Converts LHEEvent, LHEEventInfo, LHEParticle, LHEInitInfo, LHEProcInfo, LHEInit, and LHEFile to dataclasses with proper type annotations
• Adds backwards compatibility methods (getitem, setitem) with deprecation warnings for dict-like access
• Changes LHEVersion from float to string type and updates related tests

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/pylhe/init.py	Core implementation converting classes to dataclasses with backwards compatibility methods
tests/test_classes.py	Updated tests for TypeError exceptions, added backwards compatibility tests, and dataclass comparison updates
tests/test_lhe_reader.py	Updated LHEVersion test assertion from float to string comparison

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