Python-friendly return of 3-element vector from VTK class

[lassoan (Andras Lasso)]

@omdaniel

I would recommend against raising errors if the object has capacity to perform other valid computations whether or not this property is valid.

Of course I would not consider adding C++ exceptions to VTK. It just could have been an option for Python, as you suggested above: sqrt(-1) returns nan in C++ but throws an exception in Python. But it seems we all agree on returning None would be more convenient.

@omdaniel

debug session in visual studio code depending on the users config the variable explorer may automatically call the getter method for the objects in scope

This is indeed a side effect of the new VTK property style wrapping, but it should not cause major trouble.

We have been calling all Get... methods of VTK objects automatically in 3D Slicer’s Python console for years (for auto-completion) and the only thing we had to fix was suppressing error/warning messages. Occasionally we run into a crash during auto-complete due to calling a Get function in an unexpected state, but it is very rare (we come across one in every few years) and the issue is easy to find and fix.