SliceO_Class Tree structure

The classes are part of a tree. We need some information for the tree structure.

Private:	m_parent_pt (SliceO_Class *)	A pointer to the parent class (or NULL for the root class) of this instance.
	m_child_nb (int)	The number of children of this instance
	m_child_pt (SliceO_Class **)	An array of m_child_nb pointers to the children of this instance.
Public:	Parent_Get_Pt	Get a pointer to the parent of this instance.
	Parent_Set_Pt	Set the parent of this instance.
	Child_Get_Nb	Get the number of children of this instance.
	Child_Get_Pt	Get a pointer to a specific child of this instance.
	Child_Get_Cur	Get a pointer to the child of this instance that is part of the "current" path.
	Child_Delete	Delete all children of this instance.
	Child_Add	Add a child to this instance.