Increase consistency in `src/Developer` files

The files in [src/Developer](https://github.com/InsightSoftwareConsortium/ITKSphinxExamples/tree/master/src/Developer) seem to hold some inconsistencies:
- The classes in the `ImageFilterX.*` and `ImageFilterY.*` files should probably be renamed to `ImageFilterX` and `ImageFilterY`. They currently define a class names `ImageFilter`, which is also defined in the `ImageFilter.*` file.
- All files might need to be renamed to have a leading `itk*`. There are actually some inconsistencies in classes with supposedly similar patterns, e.g. `itkImageFilterMultipleInputs.*` vs `ImageFilterMultipleOutputs`.*` -not sure if intended). This will need to be accompanied with the appropriate changes to the include guards.

It is probably a good idea to know if these were intended, and if yes, make it clear the reason, as it does not seem intuitive.

Maybe the notices `Warning Fix Errors Example contains errors needed to be fixed for proper output.` that are visible in the concerned pages raises from these observations, e.g.
https://itk.org/ITKExamples/src/Core/Common/ProduceImageProgrammatically/Documentation.html

Following https://github.com/InsightSoftwareConsortium/ITKSphinxExamples/pull/316#issuecomment-999242969.