Repeated variable names in modeling exercises

[ivanightingale]

In both the 0-1 knapsack exercise

LearningToControlClass/class01/background_materials/optimization_basics.jl

Line 353 in d294ddd

# @variable(model_milp, x[1:3] ...

LearningToControlClass/class01/background_materials/optimization_basics.jl

Line 370 in d294ddd

x = haskey(model_milp, :x) ? JuMP.value.(model_milp[:x]) : missing,

and the Rosenbrock valley exercise

LearningToControlClass/class01/background_materials/optimization_basics.jl

Line 581 in d294ddd

@variable(model_nlp, x)

LearningToControlClass/class01/background_materials/optimization_basics.jl

Line 724 in d294ddd

x = safeval(model_nlp, :x),

there is a variable named x. This triggers a multiple definition error in Pluto.jl.