WIP: Replace UI with IT as index type for LevelSetNode VectorContainer

[N-Dekker]

itk.IT ("IdentifierType") is the most commonly supported index type for VectorContainer. (VectorContainer is wrapped for IT and UC, but not for UI, as index type.)

• Follow-up to pull request STYLE: Replace UL with IT as index type in itkPyVectorContainerTest.py #6800

[N-Dekker]

Still under construction. I see now: in C++, NodeContainerType also has unsigned int as index type. Probably related.

ITK/Modules/Segmentation/LevelSets/include/itkShapePriorMAPCostFunctionBase.h

Line 84 in 1a6558f

using NodeContainerType = VectorContainer<unsigned int, NodeType>;

[dzenanz]

Looks good on a glance.

[N-Dekker]

This force-pushed amend aims to address CI failures at https://open.cdash.org/tests/2723817696 saying:

  File "/Users/runner/work/1/s-build/Wrapping/Generators/Python/itk/support/template_class.py", line 527, in __getitem__
    this_item = self.__template__[key]
                ~~~~~~~~~~~~~~~~~^^^^^
KeyError: (<itkCType unsigned long>, <class 'itk.itkLevelSetNodePython.itkLevelSetNodeF2'>)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/Users/runner/work/1/s/Modules/Filtering/FastMarching/wrapping/test/FastMarchingImageFilterTest.py", line 131, in <module>
    NodeContainer = itk.VectorContainer[itk.IT, NodeType]
                    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^
  File "/Users/runner/work/1/s-build/Wrapping/Generators/Python/itk/support/template_class.py", line 531, in __getitem__
    raise itk.TemplateTypeError(self, key)
itk.support.extras.TemplateTypeError: itk.VectorContainer is not wrapped for input type `itk.UL, itk.LevelSetNode[itk.F,2]`.

[N-Dekker]

This is a potentially breaking change, although in practice, VectorContainer<unsigned int, NodeType> and VectorContainer<NodeType> behave exactly the same.