Hull: evaulate_at_zero function

[dnguyen227]

The nonlinear hull reformulation computed the perspective constant f(0) with JuMP.value(v -> 0.0, con.func). This doesnt work for GeneralVariableRef

evaluate_at_zero that goes through numbers, variables, affine, quadratic, and nonlinear expressions. This allows for dispatch in InfGDP. dispatch_variable_ref should ensure that we're only evaluating things we're confident about. Can build out other types as they come up, but this solves the issue of variables inside GeneralVariableRef erroring out when they shouldnt.

using DisjunctiveProgramming, InfiniteOpt

model = InfiniteGDPModel()
@infinite_parameter(model, t ∈ [0, 1], num_supports = 3)
@variable(model, 0 <= x <= 10, Infinite(t))
@finite_parameter(model, p == 3)
@variable(model, Y[1:2], InfiniteLogical(t))
@constraint(model, exp(x) - 3 <= 0, Disjunct(Y[1]))   # nonlinear disjunct
@constraint(model, x >= 5, Disjunct(Y[2]))
@disjunction(model, Y)

# g(0), the perspective constant of the hull row
evaluate_at_zero(exp(x) - 3)   # -2.0   variables -> 0
evaluate_at_zero(exp(x) - p)   # -2.0   finite parameter -> its value
evaluate_at_zero(exp(x) - t)   # ERROR: g(0) would depend on the infinite parameter t

reformulate_model(model, Hull())   # MethodError on master: no JuMP.value(f, ::GeneralVariableRef)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.69%. Comparing base (eca397e) to head (69f1e1b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #148   +/-   ##
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  Coverage   99.69%   99.69%           
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  Files          21       21           
  Lines        2265     2298   +33     
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+ Hits         2258     2291   +33     
  Misses          7        7           

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[dnguyen227]

@pulsipher ready for review. This replaces the JuMP.value(vref) PR on InfiniteOpt.