fix error messages for when save method doesn't work properly
add list_parts method to UI
add add_parts method to UI
add condition for when parts added equals the number_of_parts for a piece, so that a user cannot add additional parts
redirect users to main menu, in parts method of the UI
fix bug in parts where current_piece grabs the wrong piece
remove validation for part that an instrument can only have letters, ex. 1st violin should be acceptable
add counter for add_parts method in UI, so a user cannot add more parts than are available
remove part_checker method from UI
add instrument table user can search for musicians by instrument