Define or remove "csgn"

[mhogrefe]

I was browsing http://fungrim.org/topic/Inverse_tangent/ and noticed that a few formulas use a csgn function that isn't explained anywhere. Is it related to sgn? It appears in formulas/expr.py and formulas/atan.py.

If csgn is just a synonym for sgn I can create a pull request to remove it. If it's supposed to be a separate function it should be defined somewhere.

[fredrik-johansson]

Many basic functions and operators don't have definitions yet.

Added csgn definition in 59bd45e

Thanks for reporting the issue!