Edits to Firefly VAC contribution

[jusneuma]

Fixes #

• S/N and E(B-V) plots are now working
• Includes the choice to select either the MILES or MaStar version of the FireflyVAC by using a callable dictionary.

ToDo: It works when I call cube.vacs.firefly, but it doesn't seem to work when I call VACs().firefly.

This pull request:

• [x ] 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.

[havok2063]

Just gave this a look through and made some comments/questions. In the meantime, I'll try to have a look at the code and see if I can diagnose why you're seeing the other error in the tools.VACs.