Refactoring & testing utils/authentication.py

[corey-nm]

This is PR is a refactor of the sparsezoo/utils/authentication.py module, as discussed in a previous PR. It also adds unit tests for all the functionality.

Summary:

• Replaced SparseZooCredentials class with two functions:
  • _maybe_load_token which tries to read in token from a path
  • _save_token which saves a token to a path
• The logic for loading a token is much more linear now, instead of spread across multiple methods