Refactor piece subclass constants
Moved SYMBOLS to piece subclasses
Move ASSIGNMENTS to HumanPlayer class and move draw method to Board class
Move past moves tracking to Board, write en passant moving logic for Pawn
Add en passant piece-capturing logic
Broke piece subclasses into files, implemented repeated position stalemate, and implemented pawn upgrades when it reaches the back row
Added castling
Working castling
made user interface with save and load functionalities