Abstracted Fourier Series Notebook

After feedback from Beth Stemmler, I abstracted a bunch of stuff from
the Fourier Series notebook. Specifically, all of the plotting stuff and
some of the calculations are hidden from the user. These things were
converted to functions that are now contained in the file
'fourier_series.py', which is imported at the beginning of the notebook.
Split Fourier Intro, Part 1

I split up the Fourier Intro notebook. The first part of this notebook
had a simple 4 point FT and the second part was FTs of sine and cosine
function. It seemed best to break these apart so that there are not so
many things going on in one notebook. I'll need to fiddle with the
second half of the notebook before committing it to the repo.
Split Fourier Intro, Part 2

Here I made a new Fourier Intro notebook that covers the FT of simple
sinusoidal functions and their complex mixtures and the inverse FT. I
also made a new plotting function file that will eventually supersede
the original.  The FT plotting function was modified to handle the
sorting of the frequency data automatically, so I didn't have to define
it in the text directly.