Expose rxvtk::from_event in python

this new API allows to create an observable of a vtkCommand event in python.

async def Observe(observable):
    async for event in observable:
        print(event)


async def main():
    ...
    observer = ReactiveCommand(sphere, vtkCommand.PropertyModifiedEvent)
    task = asyncio.create_task(Observe(observer.GetObservable()))

    ...

    observer.Unsubscribe()
    await task


# sample output
# [<paraview.modules.vtkRemotingServerManager.vtkSMSourceProxy(0x55613949cfb0) at 0x7f097d6c7520>, 73, None]
# [<paraview.modules.vtkRemotingServerManager.vtkSMSourceProxy(0x55613949cfb0) at 0x7f097d6c7520>, 73, None]
# [<paraview.modules.vtkRemotingServerManager.vtkSMSourceProxy(0x55613949cfb0) at 0x7f097d6c7520>, 73, None]
# [<paraview.modules.vtkRemotingServerManager.vtkSMSourceProxy(0x55613949cfb0) at 0x7f097d6c7520>, 73, None]
# [<paraview.modules.vtkRemotingServerManager.vtkSMSourceProxy(0x55613949cfb0) at 0x7f097d6c7520>, 73, None]

Limitations:

• callData i.e the third argument of virtual void vtkCommand::Execute(vtkObject * caller,unsigned long eventId, void * callData) is not wrapped.
• There is no way currently of unsubscribing besides destroying the vtkObject we are observing.

There is no technical reason for it to be in ParaT class but I think it looks convenient.