inputs and outpus: always return the default values

[VEZY]

It is complicated to get the default values for a model, or a set of models. And we already have the inputs and outputs function to get the names of the variables. It is then trivial for users to only get the names on the returned tuple, so we should propagate all the information, not just the name.

Also, while doing so we should change the internal inputs_ function for inputs (same for outputs), because it can be confusing for users.

[VEZY]

I prefered doing this in variables. Fixed it in c62a693