diy fix in vtkOverlappingCellsDetector

The way block linking was handled didn't quite scale up. Non symmetrical link maps could be produced. The code has been cleaned and now relies on the more robust / easy-to-use vtkDIYUtilities::Link to link relevant blocks together.

The map was also storing nullptr ugs when blocks were not to be linked. It was adding unnecessary complexity. Now, the filter doesn't allocate anything, and relies on the existance or not of an ug for block at given global id.

Addresses paraview/paraview#20202 (closed)