Prepare for new expiration model - introduction of MultipleExpiration
Expiration base class) and
MultipleExpiration classes (to mix different expiration kinds easily), with corresponding tests.
This is a prerequisite needed for the creation of a new
Expiration class with a more complex model.
As a side product, the
MultipleExpiration class simplifies the model generator, removing the now obsolete
This MR can also be merged to master, in principle (results are not changed).