[WIP] Parse kicker when reading Elegant files

[RemiLehe]

Allow Cheetah to parse KICKER elements in Elegant files

Description

Elegant files sometimes contains Kickers:
https://ops.aps.anl.gov/manuals/elegant_latest/elegantsu158.html#x169-17000010.49
My understanding is that this corresponds to Cheetah's HorizontalCorrector/VerticalCorrector. I updated the function convert_element to enable it to parse kickers

Motivation and Context

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

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).

Note: We are using a maximum length of 88 characters per line.

[RemiLehe]

I arbitrarily put the Horizontal Corrector first, and gave it half the length. Not sure if this is the right approach.

[RemiLehe]

Maybe this could be done differently once #203 is merged.