Bug fixes and cleanup to pass flake8 in CI

[brews]

We got automated tests up and running in CI with PR #35. Once it started to run flake8 immediately flagged climate_toolbox code for a number of reasons. This PR resolves those issues.

• Add a basic flake8 configuration to setup.cfg so we focus on big and immediate problems.
• I autoformatted all package and test code with black. This isn't required right now, but it's the fastest way to get minor formatting fixes so the code passes the flake8 check in CI.
• Fix "undefined" error in climate_toolbox/utils/utils.py.
• Fix several ambiguous star imports in climate_toolbox/io/io.py and tests/test_climate_toolbox.py.
• Fix an issue with overwritten test functions in tests/test_climate_toolbox.py.
• Dropped unused variable in load_bcsd() from climate_toolbox/io/io.py.

With these changes, flake8 passes the code on my local machine.

[brews]

This PR passes now the flake8 check in CI but fails when pytest is run. This is expected and progress because pytest wouldn't even run before this. That is, the fact that this is even running is good — even if it's failing.

Fixing the new issues with this pytest check will need to come in a later PR, likely at a later date.