doc(osx-gcc.md): add pkg-config requirement

[skip ci]
feat: add build-dreal, build-dreach script
tests: add order_model.py

For two saved model files m1 and m2,

    ./order_model m1 m2

 - returns true      if m1 \subseteq m2
 - returns false     otherwise

m1 \subseteq m2 is true if for each k in m1, we have associated interval
m1[k] and m2[k], and m1[k] is a subset of m2[k].
tests(nra): update test.sh to use order_model.py
tests(nra): add expected models for SAT cases