Use ReadImage and WriteImage functions where appropriate

[dzenanz]

Setting up a reader or a writer just to use them once in a straightforward way requires 5 lines of code, and it can be accomplished by using the appropriate convenience function.

itk::ReadImage and itk::WriteImage are available since ITK 5.2.0. See https://github.com/InsightSoftwareConsortium/ITKSphinxExamples/pull/225 for an illustration of the changes required and https://github.com/InsightSoftwareConsortium/ITKSphinxExamples/issues/226 for further discussion.

This can be done piece-wise. A possible starting point are bundled examples which are used in the software guide. @josetascon did a good job in ITKSphinxExamples.