Modified Fourier Intro, Removed plotting functs

Made several textual modifications to the Fourier Intro notebook. In
addition, I had defined two plotting functions for making wide dual
plots. I used these in several other notebooks, so I removed them into a
separate plotting.py file that can be %run in each notebook. Will clean
up many of the other notebooks.
Removed plotting functions, import instead

Went through all of the notebooks, and removed the explicit definition
of the plotting functions. Instead, I do a %run on the plotting.py file
to import these functions from a global location. Made a couple of small
text modifications, and changed the colors of the FT plots in
plotting.py.