WIP ENH Add dics_source_power
No need to repeat source power to create source estimate object
ENH dics_source_power now supports lists of CSD matrices
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
ENH : addressing some of denis' comments on DICS code
avoid saving files
ENH: improve csd shapes tests
PEP8
Use and improve brain plot message
Use instance methods x.max() instead of np.max(x)
More explicit variable name

pow_max instead of amp_max, etc. better indicates that this is the maximum of
source power
Use preset limits for plotting source power
ENH CrossSpectralDensity now holds frequency

This allows us to use dics_source_power without passing a parameter with all
frequency values.