[Dev] Add parameters

FTR:
- for indirect matters, `variable` should be enough @gergaud
- for direct it is *a priori* not and there is a simple way to define parametric optimisation models, *e.g.*, in ExaModels.jl (check [this](https://exanauts.github.io/ExaModels.jl/stable/parameters) - no sensitivities, though)
- for direct, returning a function (of any parameter one wants) towards a model is easily done and may be not too harmful for performance; unless one wants more (sensitivities through suited AD)
- if necessary: extend the syntax by allowing

```julia
@def begin
p in R^r, parameter
...
end
```