Specification of output-product suffixes

[knoepfel]

Change output_products(...) to output_product_suffixes(...)

For the creator nodes (i.e., transforms, folds, and unfolds), we should make the following changes:

   m.transform(...)
     .input_family(...)
-    .output_products("<suffix 1>", ...);
+    .output_product_suffixes("<suffix 1>", ...);

to make the code agree with the CDR.

Default suffix specification (e.g., phlex::default_suffix)

We should also add a symbol (e.g., phlex::default_suffix) so that users do not have to know what the default suffix is:

  std::tuple<int, int> two_numbers();

  m.transform("make_two_numbers", two_numbers, phlex::concurrency::unlimited)
    .input_family(...)
    .output_product_suffixes(phlex::default_suffix, "the_better_one");

where the phlex::default_suffix symbol resolves to an implementation-defined string that the user does not need to know.

[knoepfel]

Re-opening as #448 must be merged before this can be closed.