bus companion blocks

[guruofquality]

Core bus support blocks

• Reference issue: the bus agenda PothosFlow#92

Goal: create a set of core blocks for dealing with bus manipulation in a programmatic way. This allows gui users and JSON markup users to manipulate buses without loops.

• bus concat - concatenate several ports or buses into a single larger bus. This is effectively a Topology block composed of only connections - no runtime overhead.
• bus splitter - split an input bus into several sub ports or buses. Also a pure topology block without runtime overhead.