Unequal time Greens/Measurements (WIP)

[ffreyer]

TODO:

• [x] Add unequal time expectation values to ED
• [x] Add general greens(k, l) with k >= l (Is l < k ever required?)
• [x] Add Iterators for faster computation
• [x] Add general tests for greens(k, l) & iterators
• [x] Add UnequaltimeGreensMeasurement
• [x] Add ED tests for UnequaltimeGreensMeasurement
• [x] Figure out an efficient way to measure (multiple) unequal time observables
• [x] Add susceptibilities (CDC, SDC, PC)
• [x] Add current-current correlations
• [x] Add Tests for LatticeIterators
• [x] Fix susceptibility tests/implementation
• [x] Cleanup
• [x] Add tests for new linalg methods
• [x] Deprecations?

On designing measurements:

The susceptibilities as well as current-current correlations take the form \int_0^\beta <O_{ij}(\tau, 0)> d\tau. After Wicks, this the observable should be require G(\tau, 0), G(\tau, \tau) and G(0, 0) if I'm not mistaken. So it makes sense to (a) have an iterator which efficiently calculates G(\tau, 0) and G(\tau, tau) (whilst keeping a G(0, 0) alive), and (b) to compute these integrals synchronously (in terms of \tau).

[ffreyer]

closes #20

[ffreyer]

Added some tests for #80.

[ffreyer]

The redesign I've started closes #32.