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.

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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