EarthDataDownloader object to retain user login credentials

[PennyHow]

Description of the desired feature: Functionality added so that login credentials are retained by the EarthDataDownloader object after first inputted, and therefore the user does not have to keep re-defining them for each download.

In the tutorials, there is repeated EarthData login requests. For example, in the 3D_stack tutorial:

measures_shelves = fetch.measures_boundaries(version="IceShelf")

Also, there is nothing in your documentation on how to define credentials as environment variables (the icepack link in the API reference for EarthDataDownloader is broken).