Agnostic loading of BROWSER and CATALOG urls

[wildintellect]

#116
Always just sub BROWSER url for STAC url. Can better describe in docs what values are expected. See also #116

Summary:

Old code handled MAAP and VEDA STAC_BROWSER_URL differently, this assumed that the ENV variables were quite different. By standardizing how the ENV is supplied agnostic code can be used, and a fallback in the event of missing variable can also be handled.

Fixes or Addresses Issue #: [ticket number & link]

#116

Checklist before requesting a review:

• My code follows the guidelines of this project
• I performed a self-review of my code changes
• I have added my changes to the change log (HISTORY.rst)
• I have completed spell-checking, removed unnecessary print statements & commented-out code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• New and existing unit tests pass locally with my changes

[sandrahoang686]

lgtm