Update GZ3D tutorial notebook

[CKrawczyk]

Karen has gone through and cleaned up some of the explanations and added a few links in places.

As part of this update we also fixed a bug in the polar_plot code where labels were not being used in the scatter plot calls. The examples in the notebook now explicitly create figure legends to ensure this is working as expected.

Fixes #

This pull request:

• Has a title that summarises what is changing.
• Updates the documentation accordingly.
• Has unit tests & code coverage is not adversely affected (within reason).
• Works with Python 2.7 and 3.6 (and ideally with Python 3.7).
• Updates the CHANGELOG.
• Removes more lines of code than it adds.
• If relevant, adds a new entry to the What's new? page.

[CKrawczyk]

@havok2063 Just some minor updates to the GZ3D code, ready to merge once reviewed.

[havok2063]

Looks good!