Introduce C++ data-product providers according to CDR

[knoepfel]

The Phlex conceptual design uses data-product providers to "provide" data products that cannot be supplied by any user-defined algorithms in the graph. The current phlex implementation does not yet have such a concept.

To implement providers:

• The registered "driver" should yield data-cell indices instead ofphlex::product_store_ptr objects.
• What is called a "head port" will be replaced with a provider.
• The multiplexer will no longer route product stores to head ports; it will route data-cell indices to providers.

[knoepfel]

The intention is to create an interface that looks similar to:

A give_me_an_a(data_cell_index id) { ... }

g.provider(give_me_an_a)
  .input_indices("spill")
  .output_product("Gauss:number");

Many providers will need to be created automatically for data products that reside in framework output files. But a "typed" interface (as shown above) will also be supported.