Cross-correlation based on fluidsim.base.output.spectra
We like to have cross-correlation spectra
This should automatically compute all cross-correlation spectra depending on the fields in state_spect.
We should save the different 1d cross-correlation spectra (versus, kx, ky and kz) and the 2d spectra (versus
The code should be strongly based on fluidsim.base.output.spectra and it may be that we don't need specialized classes for the different solvers.
Related to #87