Add a Postprocessor process to fix known errors made by models

[clemsciences]

We have errors like the ones mentioned in #1190, #1166, #1122. They could be handled by the addition of a Process class that programmers would customize in order to fix known errors made by trained models.

[polycrates]

As I understand it, #1190 is not an error from a trained model, it's a simple code error which requires a reversion. I may be wrong about that, but if I'm right, why not fix the code?