Update PolynomialsBDM.degree()

[GrahamBenHarper]

While making changes under #8588, I noticed that PolynomialsBDM defines its degree() function to return polynomial_space.degree() - 1.

The goal is to assign a degree to the base class during construction before the polynomial_space is constructed.

I imagine the best way to tackle this is to write down what polynomial_space is by hand and see if there is a formula for the polynomial degree in terms of the argument for the constructor, k.