Implement a spectral density analysis for mode identification.

[dschaffner]

Using two point correlation methods, implement a code to produce a spectral density plot (wavenumber vs frequency). Requires computing cross phase from FFT. Takes data from two spatially separated diagnostic points (Isat, Bdot, etc), computes a cross phase, then an estimated wavenumber using crossphase and separation distance, then histogram power as a function of wavenumber and frequency.

Ref paper: https://aip.scitation.org/doi/10.1063/1.1686491