Improving energy analysis.

energy_budget computes the potential enery released and kinetic
energy (as Cabot does), and would normally infer dissipation from
their difference.  When a simulation goes unstable, energy
conservation fails and this analysis becomes silly.

The units of the Fourier analysis plots have been switched to
energy density for the purposed of rho vs. velocity analysis.
The spectrum of just u_z contribution to K is also given.
Added dashed lines with slope -5/3 to compare to Kolmogorov.