Quadrature rule algebra

The proposed changes will add the capability of generating sparse grids.

The proposed changes provide building blocks of generation sparse grids. Specifically, a symbolic representation of quadrature rules is added, which supports some basic algebraic operations.

The idea is that, starting from basic quadrature rules over [-1, 1], one can form complicated rules in higher dimensions over [-1, 1]^d. Tensor product quadrature is included as a proof of concept.

Todos:

• Add more test for algebraic operations
• Add tensor product quadrature rules