uhd: unexposed usrp_sink::get_device() and usrp_source::get_device() from SWIG

* multi_usrp class is not exposed to SWIG, so function returned a SwigPyObject and caused a memory leak when Python exits
* It is not practical to expose multi_usrp to SWIG, so simply tell SWIG to ignore it
Windows compatibility fixes

* Fixed usage of Windows thread-naming API, changed minimum Windows version
* Fixed MSVC usage of isnan, round
Merge remote-tracking branch 'ncorgan/ncorgan/swig_ignore_multi_usrp' into maint
filter: relax gr::filter::freq_xlating_fft_filter QA precision
zeromq: require zmq.hpp be installed