Changed the threshold for positive intersection
Fixed float constant type
Resolved few problems with Node architecture

Resolved a very obscure circular dependency with forward declaration. Apart
from that, it makes much more sense that the node in the Intersection would be
a Solid, since you can not actually have an intersection with a group.
Adapted to new Node hierarchy
Changed the Point implementation

From now on, Point is only another name for the Vector type. This is done with
intention to simplify the operations.h and not to have duplicate code.