[get_turns][relate] handle doubled points in relate L/L and L/A

add info about the first/last non-doubled section to sectionalize
modify get_turns() to pass info about the first/last segment to TurnPolicy::apply()
modify parameters of TurnPolicy::apply()
add searching for the next non-equal point in relate L/A when the segment of Linear must be checked if it starts in the interior or exterior of the Areal
add tests for relate and within
add relate L/A to Jamfile