Debugged hinting method in gui.pde, rm obsolete

Debugged hinting method in gui.pde by replacing indexOf searches with
Card.indexIn, which compares cards not by direct equality but by objective
equality (.equals() rather than ==)
Added test for null Cards in method Triple.member() in Set.java
Added method Triple.compOf() that returns the pair of the complementary elements in the
triple to a given third