Add support for using ParameterJuMP

[coroa]

ParameterJuMP allows defining variables as parameters and then compute dual values for them. Ie if one sets the capacity of a not-extendable generator or line as a parameter one can request the dual of the max generation constraint by querying the dual of the p_nom parameter.

[coroa]

Probably as simple as replacing all occurrences of JuMP.Model with JuMP.AbstractModel, as is already happening in the new moi branch by the introduction of the ModelView layers.

[coroa]

The question is: How would the user mark a data parameter as a ParameterJuMP-parameter? The parameter provision functions, i.e. get(::Element, ...) or get(::Data, ...), could maybe check whether something should be a parameter?