JSON topology improvements

[guruofquality]

• Support hierarchies of JSON topologies. A block config could point to another JSON file which contains a JSON topology. We would just instantiate a connect another topology from this config (recursive).
• Parameterize the topology. The markup should have a list of configuration parameters. These parameters are refered to by name in the block config arguments (like variables). The configuration parameters may have a default, but they can also be varied by 1) passing in options on the PothosUtil command line or 2) passing them in through the block config (see support hierarchies).
• Make it easier to install JSON topologies into the property tree. Consider that the user may want to express the block description for the GUI (which also has a JSON representation). Installing and locating JSON resources at runtime.
• PothosUtil topology executor should have an option to dump stats at the end of the run into a json format. This completes a benchmark runner app where test sets and results can be handled in a high level language like python -- and plotted. This should really be a new issue on Pothos::Topology to dump JSON stats for all blocks in the topology.
• The Pothos::Topology should parse the GUI's JSON format as well.

[guruofquality]

This issue was moved to pothosware/PothosCore#64