Added union-find interface and first implementation optimised for quick find operations.
Added abstract unit test class for general union-find implementations to use, and extended it to test the QuickFind implementation.
Moved stack tests into stack specific package to help organise tests.
Moved stack tests into stack specific package to help organise tests.
Merge branch 'stack'
Added to QuickFind documentation, further detailing the trade-offs of the particular union-find implementation and complexity of different operations.
Modified the QuickFind documentation.
Added the quick-union, weighted quick-union, and weighted quick-union with path compression implementations of the union-find data structure, with accompanying tests.