Added vtkFiltersCore to paraview/vtk/__init__.py.

The goal is to enable some of the core filters in ParaView's
vtk module such that they can be easily access from the
Python programmable filter.

Change-Id: Iabfb360b96ae263e0fc042adf7a30bef6271568f
Merge topic '14006-add-filters-to-paraview-vtk-python' into next

21d0ec2 Added vtkFiltersCore to paraview/vtk/__init__.py.