TEIIDDES-1568: Modify hierarchy of Mapping Nodes

* With introduction of IMappingBaseNode to spi, hierarchy of nodes
  adjusted accordingly.

* MappingBaseNode
 * Addition of addChildNode method, which is the only 'add' method
   accessible via the spi interfaces. Introduced because it collects
   together all the other methods, minimizing generics and doesn't break
   existing addChild method.