support grayscale morphology operators

[Morgane55440]

currently the operators in the morphology module only do binary operations (background = 0, foreground = 255).

but when doing morphological operations on actual operations, it is often necessary to do greyscale operations, using the whole range of values available.

i would like to propose to add greyscale variants of all the currently existing functions, as well as a Mask Struct to replace the Norm arguments, allowing for user defined masks.

if that is something that seems appropriate to add, i would love to work on the implementation myself.

see the wikipedia article on grayscale morphology

[theotherphil]

That would be a welcome addition - thanks!

[theotherphil]

Implemented in #581