flat_map: support conversion of SPL tuple to/from dictionary (named tuple)

[markheger]

Involved operator code: com.ibm.streamsx.topology.functional.python/FlatMap

Primitive operator does provide different code block for various types (python object/dict/named tuple) like source, map, aggregate.

[markheger]

current code results in following error when using type hints (named tuple as output style)

src/operator/flatten_flat_map.cpp: In member function \u2018virtual void SPL::_Operator::flatten_flat_map$OP::process(const SPL::Tuple&, uint32_t)\u2019:
src/operator/flatten_flat_map.cpp:130:41: error: \u2018SPL::_Operator::flatten_flat_map_Base::OPort0Type\u2019 has no member named \u2018get___spl_po\u2019
           pySplValueFromPyObject(otuple.get___spl_po(), item);
                                         ^
make: *** [build/operator/flatten_flat_map.o] Error 1