WIP Compare CSD implementations

Several different CSD implementations (most commented out):

* Using nitime's Welch method
* Using nitime's periodogram_csd method
* With code copied from nitime's periodogram_csd method
* Using the multitaper module's _mt_spectra and _csd_from_mt
* Using only _mt_spectra (not commented out, active)
Simple CSD implementation using _mt_spectra
ENH Add multitaper mode for CSD calculation

Based on implementation in mne/connectivity/spectral.py