Update ChessPositionBasics.cpp

Reverted "set_up" to include all pawns.

Changed "piece_at" to return reference, that assignments to board --- "piece_at(square) = ..." --- are easier.

Updated "print" to give whitespace for null squares, so that spacing of
"empty empty Pawn empty empty empty empty empty" is
"    0           " instead of
"0".

Wrote "display" (see update to ChessPostion.h).