Add unit tests to dea_tools.classification module

[robbibt]

The functions in the dea_tools.classification module are now being used for operational DEA product generation. To ensure they do not break in the future, we should write some unit tests covering typical use cases for those functions.

Proposal: We add a test_classification.py test script here (https://github.com/GeoscienceAustralia/dea-notebooks/tree/develop/Tests/dea_tools) with some basic pytest unit tests.

For test data, we could upload some small sample datasets into this folder: https://github.com/GeoscienceAustralia/dea-notebooks/tree/develop/Tests/data