Output temporal spectra ns3d (and then fix ns2d)

In a file fluidsim.solvers.ns3d.output.temporal_spectra.py:

• Recording velocity signals:

• signals of the three velocity components must be recorded by static <> that are regularly spaced in the flow. Their spacing in the three directions should be an option.
• each process must be able to independently compute the positions of its probes.
• each probe must save its signals in an independent series of files
• files must remember which probe they were recorded by, for signal reconstruction.
• files must have a size limit.
• the duration of the reconstructed signal must be able to be chosen independently of the file size limit.

• Computing temporal spectra:

• probe files must be loaded to reconstruct velocity signals.
• temporal spectra must be computed from reconstructed signals.
• temporal spectra must be averaged over different probes.