Improve serialization of univariate distributions

[ManuelAlvarezC]

The current implementation of the to_dict and form_dict methods exposes the internal attribute constant_value and returns different values depending on its value.

We should change the implementation to make that all fitted distributions of the same class have the exact same serialization independently of the value of the constant_value attribute, and also have the attribute constant_value implicitly codified in the serialization.