Fix mayavi tests after upgrading to vtk 5.10.1 (old pipeline)

1. This fixes the following 2 tests: test_poly_data_reader.py and test_image_data_reader.py as the HeaderSize attribute of vtkImageReader2 was not a LongType in the traits wrapper.

2. Fixing SphereWidget enable/disable which fails test_mlab_integration.py.

3. Upgrade PLOT3DReader to MultiBlockPLOT3DReader. This turns out to be on those examples where even though 5.10.1 has backward compatibility layer, PLOT3DReader fails. 

This PR is now good to review and merge. With this all vtk and mayavi tests pass.