Use programming "type" when connecting edges

[knoepfel]

The current implementation does not use a data product's type for connecting edges between graph nodes. This is an oversight. The requirements:

• If a product specification would create an edge between two node ports with conflicting types, a runtime error should occur before any processing of data.
• A user should not need to specify the type as part of the (e.g.) "A"_in("spill") specification (or its equivalent). The type specification occurs based on the input-parameter types and output type of algorithms.

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The word "type" appears in quotes because, long term, we do not want to use the programming type, per se, but the data-product concept. For now, a "type" that is compatible between both Python and C++ should be considered (e.g., vector[int] could be represented as a List[Int] in Python and a std::vector<int> in C++).