Misc vtkExtractSelection2 improvements

This includes several improvements to vtkExtractSelection2.

1. Correctly handle composite input in RequesDataObject. Also simplies the code to easier to follow.
2. Not create SelectionOperators during iteration over blocks, instead create them before the iteration begins. This is essential, otherwise all the acceleration structures created in vtkSelectionOperator::Initialize need to be recreated per block which defeats the objective of that method.
3. Add support to extract blocks by block ids or AMR level/idx.
4. When locating points in a cell, use vtkDataSet::GetCellPoints instead of vtkDataSet::GetCell as it is a less expensive call.
5. Make NewSelectionOperator virtual so it can be overridden by subclasses. Also, gave than method a New following VTK style.
6. Update TestExtractionExpression to correct set FieldType on all nodes. Not setting FieldType is now correctly dectected as an error.