PPSD scale of colorbar

McNamara used in his paper bins of 1dB. The optional ``db_bins`` parameter creates 0.5dB bins as standard. In the conseqeuence the colorbar scaling should go up to just 15% instead of 30% to produce the same color scale.

Another question: Is there any drawback of using longer chunks of data to get a signal also for higher periods than 100s? I tried to set ``PPSD_LENGTH = 6*3600`` and ``PPSD_STRIDE = 6*1800`` and the result looks ok.