Make AbstractOPLayout <: AbstractPolynomialLayout

[dlfivefifty]

QuasiArrays.jl now has PolynomialLayout, which currently only supports Inclusion but is designed to represent general polynomials. This package has AbstractOPLayout which should be a special class of polynomial layout.

This helps make sense of how WeightedBasisLayout should work. E.g. equality can be made precise (if w1 and w2 are not polynomial and P and Q are non-zero polynomial quasimatrices then w1 .* P == w2 .* Q iff w1 == w2 and P == Q)