Increase unit test coverage of data_utils to > 95%

[cisaacstern]

Our current test coverage of climsim_utils.data_utils is 12%, as seen here:

https://github.com/leap-stc/ClimSim/actions/runs/5898192566/job/15998878076#step:5:43

That is not surprising because our only test is a smoke test for importability:

ClimSim/tests/test_data_utils.py

Lines 5 to 6 in f7d092c

	def test_data_utils_importable():
	from climsim_utils.data_utils import data_utils

We should increase unit test coverage of data_utils to establish a benchmark of functionality before attempting any refactoring, such as proposed in #17. 95% or greater coverage would be a strong basis to shoot for.