Updated Doxyfile to search for headers in include/ folder.
Changed Square::restrictValues to accept an std::set<int> instead
of an std::initialization_list.
Finished Square::testRestrictValue.
Finished the rest of Square::testSetValue().
Finished testSquare.
Changed Puzzle:squares_ from a raw array to a std::vector<Squares>.

Also deleted Puzzle::~Puzzle().
Started working on custim exception class for Puzzle.