ENH: 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
Yeah but you reminded me that I wanted to refactor this function that's
only used twice. See latest commit :)

2014-12-22 13:00 GMT+01:00 Teon <notifications@github.com>:

> In mne/cov.py
> <https://github.com/mne-tools/mne-python/pull/1671#discussion-diff-22163430>
> :
>
> > @@ -40,6 +46,31 @@ def _check_covs_algebra(cov1, cov2):
> >                           'SSP projections.')
> >
> >
> > +def _pick_data_ch(info, ch_type):
> > +    """conveniently pick a certain type"""
> > +    idx = None
> > +    if ch_type == 'mag':
> > +        idx = pick_types(info, meg='mag', eeg=False, ref_meg=False)
> > +    elif ch_type == 'grad':
> > +        idx = pick_types(info, meg='grad', eeg=False, ref_meg=False)
> > +    elif ch_type == 'eeg':
> > +        idx = pick_types(info, meg=False, eeg=True, ref_meg=False)
> > +    if idx is None:
>
> nvm, got it. it's a private function and you won't to enforce that 'mag'
> and 'grad' are selected separately.
>
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> <https://github.com/mne-tools/mne-python/pull/1671/files#r22163430>.
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