vibration_analyze.py: Don't use the peak power for D_guess

vibration_analyze.py: Don't use the peak power for D_guess

Use the power at the Nyquist frequency.  This is what I'd meant to use
back in 82cd9c1 (Massive rewrite (v 0.6) to base everything on Cython
and revamped pypiezo, 2011-04-21), but I goofed.  Luckily, the fitting
algorithm was generally able to recover from my poor heuristic, but
using a borked heuristic is not ideal.

The fixed heuristic should reduce the number of poorly fit cases, and
improve convergence efficiency for all cases.