patch for supporting multiple airspy by serial number in gr-osmosdr

Hello all, I have completed additions to airspy_source_c.cc to support using multiple airspy devices. This is done by specifying the serial number for the device (i.e. airspy=0x644064DC317C1FCD). The current behaviour of opening the first device when no serial number is specified is still maintained. Specifying airspy=0 will also open the first device found. The serial number format is a 64bit hex number with or without a leading "0x". I have attached a patch file based on the current git source, a copy of the whole CC file as patched ( see lines 93 to 112 and 122 to 129), and a screen shot of the grc block for selecting by s/n. This patch has been tested with gnuradio 3.7.9 and 0, 1 and 2 airspy devices connected. I hope this can be placed into the current sources as I know of several requests for this have shown up on various discussion boards. I place no copyright restrictions on the code... please release it however you desire. This is my first attempt at contributing to gnu radio, so please excuse any blunders in protocol I may have made. Thank you for your fine work Lawrence Glaister VE7IT Nanoose Bay BC Canada