Minor fixes to ParticleBeam.plot_distribution

[jank324]

Description

Removes (%) from unitless axes, and adds Seaborn-style passing and returning of figures and axes, allowing for easy overlaying of particle distributions, for example to compare ground truths and reconstructions. In line with that, the contour argument is also replaced with a style argument that one can set to "histogram" or "contour".

Motivation and Context

• I have raised an issue to propose this change (required for new features and bug fixes)

Closes #328 as discussed in the meeting.

Types of changes

• Bug fix (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 change)
• Documentation (update in the documentation)

Checklist

• I have updated the changelog accordingly (required).
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.
• I have reformatted the code and checked that formatting passes (required).
• I have have fixed all issues found by flake8 (required).
• I have ensured that all pytest tests pass (required).
• I have run pytest on a machine with a CUDA GPU and made sure all tests pass (required).
• I have checked that the documentation builds (required).

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[jank324]

Here is an example showing the modified plotting, where px, py and p are unitless, and it is possible to overlay two distributions in a similar way to how Seaborn odes it (though Seaborn does something more complex for full figures, but we will ignore that).

beam = cheetah.ParticleBeam.from_astra("../tests/resources/ACHIP_EA1_2021.1351.001")
other_beam = beam.transformed_to(sigma_x=beam.sigma_x * 2)

fig, axs = beam.plot_distribution(bin_ranges=None, plot_2d_kws={"style": "histogram"})
_, _ = other_beam.plot_distribution(
    bin_ranges=None,
    plot_2d_kws={"style": "contour"},
    plot_1d_kws={"plot_kws": {"color": "red", "linestyle": "--"}},
    axs=axs,
)

@jp-ga, @roussel-ryan What do you think?

[jank324]

@jp-ga ... a little reminder to have a brief look at this