Multithread some HTG filters

Improve multiple HTG filters performance:

• Threshold: multithread the method thresholding using a mask using vtkThreadedTaskQueue
• Evaluate coarse: multithread the same way
• Contour: pre-select cells properly: Cell Preselections did not work as intended, because the sign vector was initialized inside of the child loop, instead of outside. That way, the signs were never set, and always considered false because of default initialization of the bool vector.

Tests have been added for the first 2, that enable easy benchmarking if needed. The logic is the same for both, clone cursors and multi-thread the processing of each child.