Add sgnsqr external function f(X) = sgn(x)*x^2

[eslickj]

Fixes None

Summary/Motivation:

This adds a signed square function to the contributed external function demonstration.

Here is an example of where this can be useful.

Say you have a pressure-flow relationship that says the mass flow is proportional to the square root of pressure drop, where pa is inlet pressure and pb is outlet pressure. Like this: mdot^2 = k(pb - pa).

The direction of flow is forward if the pressure drop is positive or backward if the pressure drop is negative. There are both positive and negative solutions to the equation. If we use the proposed sgnsqr function, there is now only one solution to the equation, and the sign of flow matches the sign of pressure drop.

The sign squared function is still smooth and replacing square with sgnsqr still gives us solutions to the original equation. It just limits us to one correct solution instead of two possibilities. I expect there could be many situations where it would be useful to limit the solutions of y = x^2 to a single solution.

Changes proposed in this PR:

• add sgnsqr external function to the ampl_external_demo library.

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