Rename Get/Set Variadic[Named]Input[s]() by Get/SetInput() for easier reading in FunctorImageFilter

Summary

This MR proposes to rename the following methods in new filters in Functor module:

• SetVariadicInput<N>() -> SetInput<N>()
• GetVariadicInput<N>() -> GetInput<N>()
• SetVariadicNamedInput<Tag>() -> SetInput<Tag>()
• GetVariadicNamedInput<Tag>() -> GetInput<Tag>()
• SetVariadicInputs() -> SetInputs()
• GetVariadicInputs() -> GetInputs()

I also added a default value for template parameter N so that you can now call SetInput() and GetInput() to interact with first input.

Rationale

Easier to read, understand and more backward-compatible.

Additional notes

All ongoing refactoring with FunctorImageFilter will need to update their patches.

Copyright

The copyright owner is CNES 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