Reorganizing support functions into thesis_functions module

Moving support functions to a separate “thesis_functions” module
containing three files which each contain one or more functions: astro,
initialconditions, and visualization.  The “CRTBP Propagator with Two
Spacecraft with Functions” has been updated to call these functions
from the external module (much cleaner now!)

Also I’ve been launching using the syntax “python notebook --script" so
that .py script files are automatically generated when I save my
notebooks.  The __init__.py file was manually created so that the
thesis_functions folder is recognized as a module.