Prepare for new expiration model - introduction of MultipleExpiration

Introducing _ExpirationBase (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 expiration_blend function.

This MR can also be merged to master, in principle (results are not changed).