ENH: updates GNSS tec documentation, methods testing

[aburrell]

Description

Improves the GNSS methods testing, adds a TEC example, and expands general Madrigal metadata definitions. Also fully defines the GNSS VTEC metadata. Partially addresses #3 and #41.

Type of change

Please delete options that are not relevant.

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

How Has This Been Tested?

Created an example in the docs and added unit tests. You should also now receive no warning when loading VTEC data.

Test Configuration

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

Checklist:

• [x] Make sure you are merging into the develop (not main) branch
• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have linted the files updated in this pull request
• [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.md, summarizing the changes

If this is a release PR, replace the first item of the above checklist with the release checklist on the pysat wiki: https://github.com/pysat/pysat/wiki/Checklist-for-Release

[rstoneback]

Not required, and it probably makes the example too complicated for the docs, but it would be nice if we could get coastlines on the VTEC plot as well.

[aburrell]

Yeah, I decided against it because I didn't want to figure out carteopy or get carteopy questions in the future 🗺️