Squashed 'pothos-sdr/' content from commit b3d1276

git-subtree-dir: pothos-sdr
git-subtree-split: b3d127632ab5a04e30b3d08e9da85235e6f7e537
Merge commit '444601d3b5acc85543361a253b2404b2aa320ab1' as 'pothos-sdr'
pothos-sdr: added sdr blocks to the build
pothos-util: support for convert to string map
pothos-sdr: work on source initialization
pothos-sdr: filling in calls for source block
pothos-sdr: use SoapySDRConfig.cmake style
pothos-library: block call registry argument matching
pothos-sdr: work on frontend options
pothos-gui: display units in param docs
pothos-library: doc parse preview supports invalid/valid
pothos-gui: block param preview supports valid/invalid
pothos-sdr: source makes use of preview valid
pothos-sdr: cont work on frontend params
pothos-sdr: work on register probes
pothos-sdr: work on common code for source/sink
pothos-sdr: tweaks and copied in sink impl
pothos-sdr: added emit on activation
pothos: added id to label constructor
pothos-sdr: labels for source block