Overload dflow_print to allow printing dataflow elements

[LincB]

This commit overloads the dflow_print function so that it can print act_dataflow_element structs as well as act_dataflow structs, similarly to the use of chp_print to print both act_chp and act_chp_lang_t. This is useful for visualizing and debugging dataflow-based representations.

The functionality of dflow_print(FILE *, act_dataflow *) is unchanged.