Introduce a new abstract class for external function nodes.

This fixes the following bugs:

- 1st and 2nd ext fcn deriv nodes were incorrectly added to
  DataTree::external_function_node_map, because of the chaining of
  constructors.

- the following methods of FirstDerivExternalFunctionNode were not overloaded:
  toStatic(), buildSimilarExternalFunctionNode()

- the following methods of SecondDerivExternalFunctionNode were not overloaded:
  toStatic(), buildSimilarExternalFunctionNode(), compile(),
  compileExternalFunctionOutput()