Add support for nonlinear sign(x) about mathoptinterface.jl HOT 2 CLOSED

function signpower(x::Number, p::Real)
    @assert p > 0
    return ifelse(x > 0, x^p, ifelse(x < 0, -(-x)^p, zero(x)))
end

function signpower(x::AbstractJuMPScalar, p::Real)
    @assert p > 0
    return op_ifelse(
        op_strictly_greater_than(x, 0),
        x^p,
        op_ifelse(op_strictly_less_than(x, 0), -(-x)^p, 0.0)
    )
end