/*!

 * \brief Make a new USRP source block.

 *

 * The USRP source block receives samples and writes to a stream.

 * The source block also provides API calls for receiver settings.

 *

 * Stream tagging:

 *

 * The following tag keys will be produced by the work function:

 *  - pmt::pmt_string_to_symbol("rx_time")

 *

 * The timstamp tag value is a pmt tuple of the following:

 * (uint64 seconds, and double fractional seconds).

 * A timestamp tag is produced at start() and after overflows.

 *

 * See the UHD manual for more detailed documentation:

 * http://code.ettus.com/redmine/ettus/projects/uhd/wiki

 *

 * \param device_addr the address to identify the hardware

 * \param io_type the desired output data type

 * \param num_channels number of stream from the device

 * \return a new USRP source block object

 */