[algorithms][get_turn_info] fix bug reported by Jeremy Murphy on
the boost mailing list; in collinear_opposite the set_tp method
was assuming that all point types passed to it are the same; this
is not the case: there are two different point types; the problem
appeared when using point_xy in rtree and running intersects queries;
in this case get_turn_info was instantiated with two different point types,
the one used by the user and the one used by the rtree to store boxes,
which were different.
Merge pull request #119 from mkaravel/fix/set_tp-point-types

[algorithms][get_turn_info] fix bug reported by Jeremy Murphy on the boost mailing list