ENH: Add box kernel to morphological operations apps

Summary

• remove some code duplication in appBinaryMorphologicalOperations and appGrayScaleMorphologicalOperation
• add support for box structuring elements to the above
• rename structype.foo.{x,y}radius parameters to {x,y}radius

Rationale

Box kernels are very common, and they're easy to add they are already available in ITK.

Implementation Details

The refactoring was necessary to avoid having N_kernels * N_filters cases (now it's N_kernels + N_filters instead).

The radii are used by all supported kernels, so they're now unrelated parameters; this avoids some more code duplication.

Additional notes

If we ever want to support image kernels, the radii parameters will make no sense. A warning can be added in that case.

Copyright

The copyright owner is CS ROMANIA and has signed the ORFEO ToolBox Contributor License Agreement.

Check before merging:

• All discussions are resolved

• At least 2 votes from core developers, no vote.

• The feature branch is (reasonably) up-to-date with the base branch

• Dashboard is green

• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement