Output ns3d: spectra(kz, kh) and spectral energy budget

Created originally on Bitbucket by paugier (Pierre Augier)

Was already merged in Bitbucket before import, marked as merged by the import user

• Spectra kzkh
• Fix issue with MPI and kzkh_periodicity
• Documentation ns3d and ns3d.strat
• plot E(kz,kh)
• dict_arrays_1time -> arrays_1st_time
• Compute spectral energy budget for ns3d and ns3d.strat solvers

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