ENH/DOC: improve and extend examples

[dengemann]

The more urgent examples are certainly:

• downloading data
• show ref channel correction
• reproducing the HCP consortium evokeds
• showing coregistration
• computing forward models
• computing MNE inverse solutions
• computing connectivity
• computing searchlight decoding

[jona-sassenhagen]

I'll see about an example for anatomy + forward models.

[dengemann]

I'll see about an example for anatomy + forward models.

For anatomy I had in mind a tutorial on how to expand the repos and extract the BEM and plot it. Then one could compute the forward model too.

Otherwise an example on source localizing evoked reponses would be important too.

[dengemann]

And there we go, one more example: http://mne-tools.github.io/mne-hcp/auto_tutorials/plot_reference_correction.html#sphx-glr-auto-tutorials-plot-reference-correction-py