Improve stability of local_minima

[michaelbratsch]

Description

As stated in issue #7530, local_minima is not numerically stable for extremely small floats due to the inversion of the image which subtracts the values from one. Just flipping the sign is sufficient to be able to dispatch to local_maxima and does not cause any rounding errors.

Checklist

• A descriptive but concise pull request title
• Docstrings for all functions
• Unit tests
• A gallery example in ./doc/examples for new features
• Contribution guide is followed

Release note

For maintainers and optionally contributors, please refer to the instructions on how to document this PR for the release notes.

Improve numerical stability of `skimage.morphology.local_minima`
for extremely small floats.

[lagru]

Looks perfect. Thanks @michaelbratsch for making these improvements! 😊

For other reviewers, it's useful to keep using invert because it also handles dtype bool for which a simple -image wouldn't work.

[lagru]

Thanks @michaelbratsch!