Feature/bias tee

[djbarbalato]

Adding bias-tee support for RTL v3+, which drives voltage on the antenna port for active antennas. This does require a newer rtl-sdr library which is in the pipeline for fedora releases, and the fedora rpms can be rebuilt for el7 using the following:

wget https://kojipkgs.fedoraproject.org//packages/rtl-sdr/0.5.4/6.fc31/src/rtl-sdr-0.5.4-6.fc31.src.rpm rpmbuild --rebuild rtl-sdr-0.5.4-6.fc31.src.rpm

Also, the rtl-sdr library is now available in EPEL at 0.5.3, which might deprecate the RedhawkSDR/librtlsdr project.