Showerpipe

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- Added `config` attribute to `PythiaGenerator`, which gives Pythia settings flags as a dict of dicts
- Improved reprs for `PythiaEvent` and `PythiaGenerator`, displaying XML path, and showing config settings as a tree
- Ensured fixed order output for fields in structured arrays
- Added `repeat_hadronize()` function to `generator` module, to enable re-hadronising the same hard process and shower
- Improved error descriptiveness, type annotation coverage, and online documentation (with CI / CD workflow)

No known breaking changes.

Change log:

- Provided wrapper for event explicitly, and a method for copying events
- Improved docstrings and type annotations
- Removed redundant CLI features

Includes API and CLI for Monte-Carlo showering and hadronisation, specifically with Pythia.

Les Houches file manipulation enables easy reading, splitting, and repeating of event records. This may be utilised for parallel execution. Built-in MPI parallelism will be included in a future release.

The CLI enables users to define a DAG for processing the data generated by the showering / hadronisation program, so that complex pipelines may be formed. User plugins may be written, including "filters" to enable arbitrary transformations on the data, and "sinks" which terminates a branch of the DAG by writing the data out to disk.

Documentation and examples to be included in a future release.