Add __len__ method to Parameter
Parameter.value is now always a tuple when len>2
BUG-FIX: changing Parameter._number_of_elements was not updating the bounds
BUG-FIX:
- Parameter.ext_force_positive was raising an error when len(Parameter) > 1
- The connected functions were being called when the value was not changed when len(Parameter)>1
BUG-FIX: ext_force_positive was not honouring bmin and bmax
BUG-FIX: the Parameter.__twins wrongly was an interinstance attribute. Now it is a set
Add Parameter tests