User-Defined Operator support

[LebedevRI]

As far as i can tell, SCIP itself does support user operators.
It would be really awesome if that functionality would be exposed in the wrapper using JuMP's User-Defined Operators.
As it stands right now, none of the non-Comm. (MI)LP solvers provides that functionality.

Refs. jump-dev/MiniZinc.jl#76
Refs. #282

[matbesancon]

hi, indeed it would be great, if you want to take a swing at it go for it, I sadly don't have the bandwidth at the moment