ENH: Add configuration to disable Python factory loading

[tbirdso]

Allow user to disable default loading of object factory override methods in Python. This is primarily intended to assist in debug investigations where factory overrides are suspected of interfering with expected behavior.

Behavior can be configured in two lines:

import itkConfig
itkConfig.DefaultFactoryLoading = False

With default factory loading disabled a developer is responsible for manually supplying any implementations of abstract classes that previously relied on factory instantiation. For example, itk.ImageFileReader relies on factory overrides of ImageIO to read files based on their extension, so with no factories registered the developer must supply a suitable ImageIO object, i.e. :

image = itk.imread('path/to/image.mha', imageio=itk.MetaImageIO.New())

The PythonNoDefaultFactories test is added to cover the scenario where DefaultFactoryLoading is set to False. DefaultFactoryLoading is True by default and factory loading is covered by other tests.

PR Checklist

• Added test (or behavior not changed)

Refer to the ITK Software Guide for
further development details if necessary.

[tbirdso]

/azp run ITK.macOS.Python

[thewtex]

@jcfr this, along with #3331 will allow running your test pipeline in Slicer without loading HDF5 IO