WIP: add option for automated regularization to mne.cov.compute_covariance

Todo:

- [ ] allow auto separately for mutliple sensors
- [ ] show basic sensor examples / update existing examples
- [ ] consolidate API including default values
- [ ] deal with projs
@agramfort my vision was to have a more prominent example that exposes normal functionality. That's why I put this here in inverse. It's for investigating cov impact on source estimates. I'd suggest to transform the whitening example into a cov tutorial with all the metrics and plots