2-Point (diagonal) Correlators

[emerali]

also need to be able to compute structure factors

may need to add more lattice-specific functionality for the following:

• 2-point correlators need access to the lattice in order to reduce redundant calculations (may not need this during the actual run if it doesn't speed things up a ton)
• calculating structure factors requires being able to do Fourier transforms over the lattice
• we need to be able to translate site indices into site coordinates and take boundary conditions into account