See https://github.com/usnistgov/teqp/issues/9

To avoid circular dependency in what is to come

Just need to fill in the remaining terms and test, test, test

More testing is still needed to identify other issues

I am confused...

And add test

Stopped iteration if dx values are all < 0

And to python interface

Hours later...

Oddly this fixes the error in derivative, but it should have no impact