gr-uhd: added tagged stream support
uhd: fixed tags_demo
uhd: changed length_tag member variable from string to pmt symbol
uhd: added notification for tagged stream preemptions/gaps
uhd: added length tag name parameter in GRC
uhd: modified tag preemption/gap handling
uhd: Added retune-by-tag and a command interface via message passing
uhd: Allow post-burst tune; tag handling less restrictive
docs: Updated uhd to reflect tag changes.
uhd: Added freq hopping example, improved tag handling in usrp_sink
uhd: Used uhd-internal fragmentation handling, bursty tx w/o time tags now no longer uses time specs