Changed non zero count method in PowerSpectrum to one available in all versions of numpy.
Changed non zero count method in PowerSpectrum to one available in all versions of numpy.

mjabri/topographica,ioam/topographica,Tasignotas/topographica_mirror,mjabri/topographica,Tasignotas/topographica_mirror,Tasignotas/topographica_mirror,mjabri/topographica,ioam/topographica,ioam/topographica,Tasignotas/topographica_mirror,ioam/topographica,mjabri/topographica,ioam/topographica,Tasignotas/topographica_mirror,mjabri/topographica