Allow PyConf component properties to declare data dependencies

If a value of a component property implements the data_dependencies method, PyConf will now propagate the return value of that method as inputs to the component.

This allows for properties to accept data handles, rather than just Algorithm and Tool objects, and for these to be hoisted up to the owning component.

The canonical use case is functors. An algorithm configuration might look like:

pvs = make_pvs()
Filter(Cut=MINIPCUT(Cut=1.0, Vertices=pvs))

where make_pvs is some function that returns a DataHandle to a vertex container.

Closes Moore#51 (closed).