color retrieval from texture now completely implemented, but not activated

All standard Shapes, as well as Obj, implement computeColorFromTexture(). It
was necessary to make minor adjustments in KDTree, BBox and Vector: mainly
signature const-ref-ication and new methods.
activate new texture model in light.hh, and therefore add -DNDEBUG in CMake

Probably because the 'intersection' vector provided by illumination() is not
always in the intersected Shape, assert()ions in computeColorFromTexture() are
triggered (experimented only with Triangle). However, on tested scenes
(without textures), the resulting image is the same as with the old model
(retrieving the color of the texture at point (0,0)).

Going to unit-test Triangle::containsPoint() (used in the
Triangle::computeColorFromTexture()'s assert) to ensure the problem comes from
the imprecision of 'intersection'.