One style for all notebooks

[cwehmeyer]

We still have some variance in coding style or variable/function naming and I think we should bring this in line with the rest of the notebooks:

• [x] 06 - Expectations and Observables (via bfd9f5f)
• [x] 07 - VAMP score based feature selection
• [x] 08 - common problems & bad data situations

[thempel]

What's the reference? NB 1 and 2?