[Paraview] Custom Python plugin with multiple input ports

[Utkarsh Ayachit]

> The dark side of this patch is that you are limited to one or two inputs
> ports....it would be nice to have more flexibility. It would be nice to
> define the number of input ports directly in the XML plugin file.

Yes indeed. This was just a quick hack to demonstrate the
functionality with minimal code changes. The actual solution will be
more general, for sure.

Utkarsh