obspy.signal.psd.psd / matplotlib.mlab.psd

Hi all,

there are some changes in default parameters for power spectral density
calculations in matplotlib.mlab.psd (and thus matplotlib.pyplot.psd,
too) with matplotlib version 0.98.4.
I wrote a wrapper in obspy.signal.psd.psd that -- regardless of
matplotlib version -- emulates the behavior of recent matplotlib
versions (scaling for onesided fft, scaling by sampling rate). I also
added tests to compare the results against PITSA.

For more information see:

- http://docs.obspy.org/packages/auto/obspy.signal.psd.psd.html
   (page will get updated overnight)
- https://obspy.org/changeset/2391/obspy