add validation of name for musician
add validation of instrument for musician
add validation of title for piece
add validation of composer for piece
add validation of number_of_parts for piece
add validation of instrument for part
add letter only validation for name of musician
add letter only validation for instrument of musician
add letter only validation for instrument of part
add letter only validation for title of piece
add letter only validation for composer of piece
rework validations in Piece so that the opposite works (true, too)
add integer only validation for number_of_parts for a piece