[Paraview] PythonProgrammableFilters Multiple Input Ports

[Utkarsh Ayachit]

> Using the  `input_ports="2"` attribute did the trick!

Glad to hear.

> The changes in your merge request have me really excited! Would this make it possible to make a class like `class PythonSuperquadricSource(VTKPythonAlgorithmBase)` in a .py file and load that as a plugin at runtime? If so AWESOME!

Indeed. In fact, your py file can have multiple classes and all of
them will get added, be they filters, sources, reader or writers. The
MR is pretty close to final, I just need to add more docs, examples
and missing decorators. I imagine we will continue to add more
decorators as time progresses, but I want to have an initial set that
covers common use-cases.

Utkarsh