[Test] Added helper function to prepare the input data for execute_onnx and changed the way bipolar values are handled in single fclayer modelwrapper

Merge branch 'feature/exec_hlslib_streamingfclayer' of https://github.com/Xilinx/finn into feature/exec_hlslib_streamingfclayer

wrapper attributes must now be declared in the get_nodeattr_types
and accessed using the getter/setters here, all data stored in
ONNX node attributes.