Improving Helix constructor to be more correct

[john-science]

Description

The constructor of the Helix class is a bit off. The default value for mult should be 1.0. And it does not make physical sense to try and fully define the geometry of a helix without defining non-None values for: od, helixDiameter, and axialPitch. So all three of those are now mandatory, with no defaults.

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Checklist

• The code is understandable and maintainable to people beyond the author.
• Tests have been added/updated to verify that the new or changed code works.
• There is no commented out code in this PR.
• The commit message follows good practices.
• All docstrings are still up-to-date with these changes.

[john-science]

This PR will close #533