TEIIDDES-1568: Modify hierarchy of Mapping Node SPI

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

* IMappingBaseNode
 * 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.