Option to specify variable values when determining if an inequality is active

[Robbybp]

Currently there is no option. We just use the latest solution (via JuMP.value) and let JuMP throw its error if there is no solution available. Some options:

• Methods with point::Function and point::Dict{VariableRef,Float64}. This mirrors the JuMP.primal_feasibility_report API. The skip_missing optional argument might be nice to copy as well
• A var_value::Function optional argument. This mirrors the JuMP.value API, which we probably have to use anyway

Since we probably want to use the var_value option to JuMP.value, a function seems like the most natural first thing to support. If the user has a dict, they can always just pass in x -> point[x]. Then we probably want to support skip_missing as well.