Gradient Refactor V3

[ripytide]

I think I made a mistake in #601 by changing too much at once. This time I've focused on some of the core changes and discarded some others (such as no kernel trait this time).

Summary:

• Moved associated filter() method to free-standing filter() function
• Added a filter_clamped() function which was what filter3x3() was doing previously
• Removed filter3x3() since filter_clamped() is identical and works with any dimensions
• Moved all kernel constants to the kernel module as associated constants of Kernel rather than free-standing constants
• Removed sobel_vertical() and similar functions as they are simply applications of filter_clamped() with different kernels which is not very maintainable as the number of different kernels grows.
• Added FOUR_LAPLACIAN_3X3 and EIGHT_LAPLACIAN_3X3 kernel constants.

[theotherphil]

This broadly looks good to me.

The two main changes I'd like are:

• To move the kernel constant (Sobel, etc.) back up to module level.
• Reinstate the deleted functions except for Kernel::filter, and optionally mark them as deprecated - I'm a bit wary of causing user churn for the sake of a few one-line functions.

[ripytide]

I've implemented the requested changes, are the deprecation warnings ok?

[theotherphil]

Looks good to me, thanks!

Just needs the version updating to 0.26 in Cargo.toml to get the semver check to pass.

[theotherphil]

Third time lucky! Thanks - I think exposing filter and filter_clamped this way is a definite improvement.