Cleaned power spectrum of methane has no peaks

[jkblc]

In my pull request #56 the plot has no peaks and I had difficulty getting any interesting features across many different data handling methods and plotting methods. I was not able to use the functions from preparation.py as it would have given incomplete data that could not be plotted no matter how it was handled.

[laserlab]

We discussed that problem in class. You have to check the preparations.py file function what the problem is.

[jkblc]

I'll look into that