Work-set clustering

# Added

• Documentation about integration tests in test/README.md and test/resources/README.md

Changed

• When updating clusters, descriptive keys of the initial clusters are now an optional parameter. This allows the reuse of existing clusters, even when they contain overlapping elements (https://github.com/kbrbe/work-set-clustering/issues/9)
• Integration tests are restructured: instead of a single TestClustering class with test functions, we have now different "sets" of test functions (test functions grouped in a class). Actual test cases can inherit the test function classes that are appropriate for the given context