update formatting of tutorial notebooks

[dshoman]

The updates include the following changes to the notebooks (designed to improve the presentation on the documentation pages):

• changed the headers which should address the issue in the HTML on the documentation pages
• added a little information on the external scripts (specifically: where to find them, that will be less obvious to users that are viewing the notebooks from the doc pages rather than in the directory within the BXA install)
• modified the filenames which seem to be causing problems when running fakeit, removing dashes. I am not able to reproduce the error it throws (***XSPEC Error: Invalid chars in file name), so I am not sure this will solve it; however, I cannot see any other possible issue with the filenames (the path is created using os.path.join, so there should also be no issue switching between Windows and Linux notation)