ENH Return list of CSD matrices for multiple frequencies
compute_csd returns list of frequencies of interest
Set frequencies in the returned SourceEstimate
Improve logging for dics_source_power and compute_csd
Scale frequency values to display them properly

This seems to be necessary so that the frequencies are displayed correctly when
using stc.plot. That's because scaling between seconds and miliseconds is
performed somewhere.
Add example for dics_source_power
Rename compute_csd to compute_epochs_csd
Small improvements to documentation