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
I am not sure of what to do with this example. It's valuable but complex and takes some times to run... shall we create a example subfolders for "advanced examples" ?

wdyt?

@Eric89GXL @christianbrodbeck @t3on @mluessi @mainakjas any opinion?