Topology() constructor from JSON description

[guruofquality]

given a file or a JSON string, parse to discover a list of blocks, block properties, and connections to create the topology.

//! make a topology given JSON description Topology(const std::string &json);

• We will need to document a format for the JSON (model after pothos-gui pth file)
• Blocks will be evaluated using the existing blockeval in pothos-util
• This can be used to create hierarchies and top level designs which can be executed.
• Add an option to PothosUtil to create a Topology given a JSON file and execute it

[guruofquality]

https://github.com/pothosware/pothos/commit/a4e68c287bf209e3a52cec40f4c3fe3cedc62ca5