Add new generic add method

[bakpaul]

This method's goal is to reduce as much as possible thee data serialization

[damienmarchal]

I suggest to add a second signature so we can pass real type instead of string.

py::objectadd( Node*self, py::type type, constpy::kwargs&kwargs)
{
if(issubchild(type, BaseObject)){
... }elseif(issubchild(type, Node)){
....
}
}

But this could be in a second PR.

[damienmarchal]

Shouldn't we consider to add a name generation mechanism (with +1) to avoid having name duplicated.
Of course this have a non linear performance hit in case of gigantic scene... but I would be in favor of having a robust normal add() and a second one considered "fast mode" for expert users.