Cross-correlation based on fluidsim.base.output.spectra

We like to have cross-correlation spectra E_vx_vy(k).

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 (k_h, k_z)).

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

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