Closure registry enhancement

[GabrielMouttapa]

To avoid error of triangular importations, these three parts of tunax have their own modules

• definition of abstract closure classes for parameters and states in closure.py
• definition of the different closures from these classes in closures/...py
• definition of the closure registry which is a dictionnary of these closures inclosures_registry.py
  To add a new closure the user has to add it's code in a module in he folder closures/ and then import them in the closure registry manually.

It would be nice to have a dynamic way to define the closure registry, maybe there is a solution to define it in closure.pyin order to avoid a large number of modules.