Updated image boundaries

[aburrell]

Description

Updated the IMAGE boundary files, which includes:

• new combined IMAGE boundary files,
• equatorward auroral boundary files, and
• more robust fitting method and more data.

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

How Has This Been Tested?

Added new unit tests and ran existing unit tests locally.

Test Configuration:

• Operating system: OS X Mojave
• Version number: Python 3.9
• Any details about your local setup that are relevant: develop branch of pysat.

Checklist:

• [x] Make sure you are merging into the develop (not main) branch
• [x] My commits are formatted appropriately (following the SciPy/NumPy style)
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] Any dependent changes have been merged and published in downstream modules
• [x] Add a note to Changelog.rst, summarising the changes
• [x] Add yourself to AUTHORS.rst

[coveralls]

Coverage decreased (-7.2%) to 90.771% when pulling d859b03b946ce164638a8b8de5821477bd98a9dc on updated_image_boundaries into 2ac914d8f4f21b2e99f7728a6ac0bdb1e36e2f8e on develop.

[aburrell]

Drop in coverage due to failure of ssj_auroral_boundary integration tests, as one of the ssj_auroral_boundary functions is no longer working (see https://github.com/lkilcommons/ssj_auroral_boundary/issues/12).