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
I feel this example is great to reproduce the paper but it's not the one you want to point people to in order to learn how to use this tool.

we need to modify a tiny script that almost just computes the covariance in an automatic way.