implement stuff from the tutorial

[kalenkovich]

I think it is a perfect place to start from. @dominikwelke, let's discuss whether we agree on the exact steps from the tutorial that we would like to get implemented in the package. I'll then create issues for each of them and we can do them one by one.

• calculating psd from an mne.Epochs (there is already a PR convert mne epochs to psd #4),
• calculating snr,
• basic plotting,
• TBD.