Update RC files instead of overwriting them

[garlic-os]

Description

This changes the behavior RAiDER.models.credentials.check_api to only update the one set of credentials in a .netrc file pertinent to RAiDER's operation instead of overwriting the file.

Motivation and Context

Fixes issue #638 where other credentials in .netrc can be lost.

How Has This Been Tested?

Unit tests were added that excerise check_api for every supported weather model to ensure check_api is behaving correctly, and crucially, that it no longer modifies unrelated .netrc credentials.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• [x] I have added an explanation of what your changes do and why you'd like us to include them.
• [x] I have written new tests for your core changes, as applicable.
• [x] I have successfully ran tests with your changes locally.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.

[jlmaurer]

@jhkennedy do you want to take a quick look at this before I merge?