Basic steps implementation

[dnguyen227]

• apply_basic_step(model, [Y1, Y2, ...]) replaces the input disjunctions with their product: one new disjunct per combination, containing the union of the parents' constraints
• Global constraints can be intersected into every product disjunct
• Original logical vars Y are kept and linked via Y[i] in Exactly(w_slice) → reformulates to y_i = sum_j w_ij, so the model is equivalent.
• relax_products = true makes the product binaries continuous [0,1] (the linking equalities force them integral), so binaries don't multiply (Pyomo doesnt do this, its valid according to the lit).
• Right now, basic steps is destructive, as this all takes place before the user calls optimize and triggers the reformulation workflow.
• Also InfiniteGDP implementation (it doesnt seem to benefit solve times at all from my minimal testing).

model = GDPModel(HiGHS.Optimizer)
@variable(model, 0 <= x[1:2] <= 10)
@variable(model, Y[1:2], Logical)
@variable(model, Z[1:2], Logical)
@objective(model, Max, x[1] + x[2])
@constraint(model, x[1] <= 1, Disjunct(Y[1]))
@constraint(model, 2 <= x[1] <= 3, Disjunct(Y[2]))
@constraint(model, x[2] <= 1, Disjunct(Z[1]))
@constraint(model, 4 <= x[2] <= 5, Disjunct(Z[2]))
d1 = disjunction(model, Y)
d2 = disjunction(model, Z)
g = @constraint(model, x[1] + x[2] <= 6)

# replace d1 and d2 with their product disjunction (2x2 = 4 disjuncts),
# intersecting the global constraint g into every product disjunct
bs = apply_basic_step(model, [d1, d2], constraints = [g], name = "bs")

optimize!(model, gdp_method = Hull())   # tighter relaxation, same optimum

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.54%. Comparing base (a4ebe8a) to head (e3005b4).

Additional details and impacted files

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##           master     #141      +/-   ##
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+ Coverage   99.51%   99.54%   +0.03%     
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  Files          17       18       +1     
  Lines        2061     2204     +143     
==========================================
+ Hits         2051     2194     +143     
  Misses         10       10              

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