only transformations are callable

[scifuentes]

restricts the ability to do cube(3)(cube(2)) by subclassing the transformation objects and moving there the call method previously present in OpenSCADObject

• while not a bug -this was legal and the code accepted by OpenScad- it was misleading since the second cube will never show

[scifuentes]

While initialy simple, to keep the code coherent, the consequence of allowing only the transformations to be callable is that only transformations need to have childrens and all what is associated with them ... therefore all code dealing with childrens have been moved to the transformation class