ENH CrossSpectralDensity class and compute_csd

* Class for storing cross-spectral density, analogous to Covariance
* compute_csd computes cross-spectral density from epochs
Add compute_csd to __init__
Use compute_csd in example

* Example now uses epochs instead of raw data
* No longer doing whitening so that noise normalization is done as it is
  usually done in DICS (dividing by projected noise)