Fixed mix-up between rank/file, work toward determining valid moves for a piece
- was using rank/file wrong - now, they're each enums, with the values being the possible ranks and files
- removed error-handling from bad rank/file since now enums - higher level makes sure valid
- started validMoves method that determines which cells a piece can move to
  - implemented validMoves for white and black pawns
  - tested validMoves for white and black pawns on an empty board
  - more tests needed