Preprocessor update

— Bug fix: allow external functions in model local variables (also add corresponding
  integration test)
— JSON output:
  + do not append two underscores after model local variable names in their
    definition
  + when writing the model after computing pass, use TEF terms for (not
    derived) external functions
  + write model local variables in parsed model (modfile.json) (Closes: #1723)
— In the driver file, provide mapping between model local variables and indices
  in the temporary terms vector (Closes: #1722)