unit of population-rate power spectrum

[tomtetzlaff]

At several instances of the code, the unit of the population rate power spectra appears to be wrong, for example in

https://github.com/INM-6/nnmt/blob/master/nnmt/lif/exp.py#L1785

or

https://github.com/INM-6/nnmt/blob/master/examples/bos2016/power_spectra.py#L123

This may be inherited from an older version of the code. See

INM-6/lif_meanfield_tools#65

The unit is correctly documented:

https://github.com/INM-6/nnmt/blob/master/nnmt/lif/exp.py#L1813