Allow printing of OptiNodes and OptiGraphs

[trulsf]

During development, I often find it useful to print the model in the REPL, When developing a Plasmo model incrementally, I got errors when doing this for specific OptiNodes and OptiGraphs. It seems just to be some missing pieces of the JuMP interface that is missing.

For e.g. getting OptiNodes printed, I needed to add the following two missing functions:

# Ensure that nodes can be printed as a normal JuMP problem
function JuMP.objective_function_string(mode, model::OptiNode)
   return JuMP.function_string(mode, objective_function(model))
end

function JuMP.constraints_string(mode, model::OptiNode)
   strings = String[
       constraint_string(mode, cref; in_math_mode = true) for
       (F, S) in list_of_constraint_types(model) for
       cref in all_constraints(model, F, S)
   ]
   return strings
end

Is this something worth including? I could probably find time to do a PR if you like.

[trulsf]

This issue was meant for the general Plasmo.jl repository. Nice if someone with access could move it.