Add set conversion bridge

[blegat]

This is useful in constraint programming where you often have a set that is a special case of another one. One example is CBLS.jl that has sets with a few more options that the MOI ones: https://github.com/JuliaConstraints/CBLS.jl/pull/37

[odow]

This need docs and tests

[odow]

I'm still not convinced this is necessary

[odow]

Isn't a better solution just for CBLS to use the MOI sets?

[blegat]

CBLS defines some with additional options so if the user uses the MOI set than he can use MiniZinc as well but if he plans to use CBLS only then he can uses sets with addtional options