Fix inconsistent naming of poly classes?

**Application**
The class naming of the polynomial classes seems inconsistent to me ("model" vs. "polynomial"). For example, consider the higher order polynomial class (`BinaryPolynomial`) vs. the quadratic one (`BinaryQuadraticModel`).

**Proposed Solution**
How about `BinaryPolynomialModel` for the higher-order polynomial class? The documentation even uses this terminology already! ("Higher-Order Models")