Improve guide's broadcast example

[Dan Bonachea]

The current broadcast example in the programmer's guide is very artificial and could use improvement.

Also it lacks a backing code file to enable regression testing of the snippet.

[Dan Bonachea]

example/prog-guide: Improve the broadcast example

It now demonstrates how to efficiently broadcast a std::vector whose size is initially known only by the root rank.

Resolves issue #575

→ <<cset 744a2b1c037e>>