Gradients, Filter and Kernel Refactor

[ripytide]

• Made Kernel::filter a free-standing function which is more consistent with the rest of the library
• Made more functions use the Kernel type for better dimensionality checking
• Added some asserts for functions that take specific dimension kernels
• Added GradientKernel enum for common gradient kernels which replaces all the pub static HORIZONTAL_SOBEL and similar (and adds a new Roberts kernel)
• Removed horizontal_* and vertical_* functions since they can all be achieved using filter with GradientKernel
• Added a SeparableKernel type for use with the gradient function to make function parameter passing more concise.

[ripytide]

closing as this has been reworked in a V2 #601