fix the one massive bug I've found: gap_num is wrong on copied boards
move board creation to it's own method
add tile_string method and make repr better spaced
generalise find_gap into find(num)
add possible_futures method
fix couple mistakes and make hash method magic
add score method
add scramble method
remove excess prints from Solver
put use of Solve class intp brute_force function
fix repr spacing in doctests
add a* search
allow brute_force and a_star to compete on same board