Add new overload of PointCloudColorHandler::getColor()

[taketwo]

Add PointCloudColorHandler::getColor() that returns VTK data array and deprecate old getColor(vtkSmartPointer<vtkUnsignedCharArray>&) in its favor. Follow-up to the discussion in #3176.

In the process of implementing my propsal from #3176 I realized that we can not change the signature to use vtkSmartPointer<vtkUnsignedCharArray> since some of the handlers actually produce arrays of floats. However, the idea to always allocate data array inside getColor() and return it as a smart pointer was implementable, so here we go.

Note the two default implementations of getColor in the base that are provided for backwards compatibility. They use each other, which may seem like a circular reference, however in practice should not be a problem. The reason to implement them this way is to support two use-cases in downstream projects:

1. Custom user-defined handlers. Such handlers certainly override the "old" getColor (because it was pure virtual, thus required to be overriden). Therefore the "new" getColor calls these implementations, and there is no circular reference.
2. Usage of now-deprecated getColor in user code. In case it's a PCL color handler, the default implementation of "old" getColor will call the new getColor, which is overriden in every PCL color handler, so again no circular reference. In case it's a user color handler, according to item 1 it will again have its own override of the "old" getColor, so no circular references.