Return the turn at $n in the form

  1     left turn
  0     straight or 180 reverse or no move at all
 -1     right turn
undef   no turn exists at $n

The path is taken to go in a line from $n-1 to $n and the turn is then whether $n+1 is left, right, or on that line. $n can be fractional. If there is no X,Y for any of the three points considered then the return is undef.

Return the minimum or maximum LSR value returned by $path->n_to_turn_LSR($n) for integer N values in the path. If there are no turns at all then return undef.