Add interactive elements to the tutorials

[tompollard]

We should consider adding interactive elements to the tutorials, so that running through the content involves more than copy-pasting. e.g.:

• Load record x. How many channels does the record contain?
• Use the WFDB plot function to plot the PPG signal.
• What happens to the signal at point t?
• Plot the ECG channels alongside the PPG. How would you interpret this event?
• etc..