pothos: added id to label constructor
pothos-gui: display units in param docs
pothos-library: doc parse preview supports invalid/valid
pothos-gui: block param preview supports valid/invalid
pothos: default to tcp for network iogress

udt was the default, but there was random unit test failures.
Not sure about the cause, but tcp should be a safe default.
We can fix and profile udt in the future.
pothos-library: fix topology commit throws
Merge branch 'master' into sdr_support
pothos-sdr: fix emitActivationSignals per channel signals