add support for relative step sizes

[FFroehlich]

No description provided.

[dweindl]

👍

[dweindl]

Suggested change

	warnings.warn(f'point is zero valued in dimension {dimension},'
	f'resulting in zero step.')
	warnings.warn(f'point is zero valued in dimension {dimension}, '
	f'resulting in zero step.')

I am wondering if this could be handled better. Provide a fall-back absolute step in case the step would be zero? Interpret the supposedly relative step as absolut? Fine as is for now.

[dweindl]

@dilpath Which of the tests are expected to pass? I think the ones expected to fail should either xfail, or be moved to a separate pull request.

[dilpath]

Thanks for the various PRs @FFroehlich! Unfortunately, there is a redesign of this package (the redesign branch, examples in [1]), which has various improvements, and I was waiting for feedback on it via #16 before merging. However, I didn't anticipate others submitting additional PRs in the meantime.

I intend to incorporate the work done in your PRs into redesign. Is there a time-critical element to this -- would you like to have these PRs merged into main soon? If so, I can review them now, ignoring the redesign for now.

[1] https://github.com/ICB-DCM/fiddy/blob/redesign/examples/derivative.ipynb

[dilpath]

done in #20