Generify and rename HTG filter VisibleLeavesSize to GenerateFields

Filters/HyperTree/CMakeLists.txt

Filters/HyperTree/vtkHyperTreeGridGenerateField.h

Filters/HyperTree/vtkHyperTreeGridGenerateFieldCellSize.cxx

Filters/HyperTree/vtkHyperTreeGridGenerateFieldCellSize.h

Filters/HyperTree/vtkHyperTreeGridGenerateFieldValidCell.cxx

Filters/HyperTree/vtkHyperTreeGridGenerateFieldValidCell.h

Filters/HyperTree/vtkHyperTreeGridGenerateFields.cxx

Filters/HyperTree/vtkHyperTreeGridGenerateFields.h

Filters/HyperTree/vtkHyperTreeGridVisibleLeavesSize.cxx

Filters/HyperTree/vtkHyperTreeGridVisibleLeavesSize.h