Rewrite the DFT notebook (#243)
Rewrite the DFT notebook (#243)

* Rewrite the DFT notebook

* theoretical background for the discrete fourier transform
* Analysis of a superposition of periodic signals
* analysis of noise, polynomial and truncated periodic signals
* conclusion with the main points at the end
* after submission, an issue will be opened to incorporate
normalization of FFT coefficients in tsfresh

* Incorporate comments

* plotted the raw signal with another linestyle
* discarded the explanation of the symmetry which results in a
splitted amplitude but added it to "further research"
* added insight that 0th coefficient equals the sum of series
values

* Change metadata kernelspec to Python 2

* The Travis test should pass now

* Modify conclusion with Max' suggestion

blue-yonder/tsfresh,blue-yonder/tsfresh