Reports problems found in Comparator.compare() implementations:
- Warns if parameter is not used. Most likely this is the result of a typing mistake and one parameter is compared
with itself or the method is not implemented correctly.
- Warns if it's evident that comparator does not return 0 for same elements. Such comparator violates the contract
and result of sorting could be unpredictable if equal elements are encountered. In particular,
sorting method may fail with exception on some data sets.