Implement DefineFunction management scheleton.

DefineFunctions are similar to DerivedFields, since both make usage of EXPRESSION.

Differences are

1) DerivedFields methods should referenced inside "transformations" map, DefineFunctions not

2) DerivedFields methods names are automatically generated, while DefineFunctions ones are retrieved from DefinedFunctions declaration

3) DefineFunctions requires at least one Parameter (beside the List of already defined fields  )