skimage.filters: forward and compute_hessian_eigenvalues should be public API?

[grlee77]

Description

The file skimage/filters/lpi_filters.py defines the class LPIFilter and functions forward and inverse, but for some reason forward is not part of the public API listed in the filters module's __init__.py. This forward function was also not in the init's __all__ prior to the introduction of the lazy loading PR, so I don't think that PR is to blame.

Isn't this an oversight? I don't see why we would have inverse be public, but not forward. On a related note, forward and inverse are not very descriptive function names in that there is no indication what type of filter these apply to! Perhaps with revisiting the API for these two for the 1.0 release.

On a similar note ridges.py defines a function named compute_hessian_eigenvalues that used to be imported to the top-level skimage/filters/__init__.py, but was also missing from __all__ and is now not available via lazy loading.

[grlee77]

Neither of these functions has standalone test coverage, so perhaps not intended to be public API?

compute_hessian_eigenvalues appears to be used internally by meijering, sato and frangi, but is not otherwise used anywhere else in the tests.

[stefanv]

Re lpi interface, see also #6418

[lagru]

compute_hessian_eigenvalues was never intended to be part of the public API. May cause breakage for a small number of users but @stefanv thinks that is acceptable. #6418 addressed the other concern.