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

Todo:

- [x] show basic sensor examples / update existing examples
- [x] consolidate API including default values
- [x] deal with projs
is there an easy example? I would just expect something like the user run auto and return either all the tests or just the best cov and the standard cov that we have been using, and then have a plot showing the contrast of two on whitened data to show the difference it makes.