Merge pull request #20 from mborne/master

ST_Extrude et Intersection
[test]add test for algorithm::signedArea(LineString|Triangle)
[convexHull]restore triangle for 2D convexHull, add missing test case for 2D convexHull (point,c olinear, triangle and polygon), fix problem with empty in 3D
Merge branch 'master' of https://github.com/Oslandia/SFCGAL
update todo for extrude
[algorithm::force3D]add tests and complete comments for doxygen, fix behavior for empty point
[algorithm::force3D]add tests and complete comments for doxygen, fix behavior for empty point