Import USRP support.

Add LO offset support.
Only set tx/rx_bandwidth after getting tx stream, to reduce TX LO leakage for <10MHz bandwidths.
Check for reference and LO lock before getting streams.

plugins/samplesource/usrpinput/usrpinputsettings.h

@@ -37,6 +37,7 @@ struct USRPInputSettings
     // global settings to be saved
     uint64_t m_centerFrequency;
     int      m_devSampleRate;
+    int      m_loOffset;
     // channel settings
     bool     m_dcBlock;
     bool     m_iqCorrection;