[disjoint] added missing namespace
[distance] segment-segment distances are now disactivated by default; can be activated by means of the macro
BOOST_GEOMETRY_SUPPORT_SEGMENT_SEGMENT_DISTANCE
[distance] more efficient implementation of segment-to-box distance
[distance] add missing include
[disjoint] true disjoint implementation (instead of intersection implementation); done to avoid double negations;
[test][distance] remove unused/commented code; add more test cases for the new 2D segment-box distance computation;