Made legalMovesFilter protected.
Adding test for en passant pawn move.
Adding PawnTracker for posterity.

Changing to an easier approach for en passant but just a very bare reminder in
case we want to visit the idea of a PawnTracker in the future.

(My rough estimate is that PawnTracker is lighter in memory without sacrificing
EnPassantFilter speed but at the cost of an extra book keeping step. But at this
point this is all just premature optimization.)
In the process of using moveList.
Adding an initial null-sentinel to movelist.
Adding public reference field PIECE_NAME to Pawn.

I'm thinking if this is a better way to checking the nature of a ChessPiece as
opposed to constructing a similar ChessPiece object and using the equals method.
Done with EnPassantFilter but it is not yet working properly.
Adding En Passant filter. Partial work for #3.