Implements TDD unit tests for getUniquePlan
Modifies DataSet match function with onlyUnique bool
Implements getUnique plan functions for Item classes and associated functions
Implements onlyUniqe option in DataSet::match
Adds book-keeping and additional addCell functions
Adds function to find non-match header indices in other data set
Implements merge function, with updated unit tests
Implements union, difference and intersection functions