Various fixes and enhancements
- AM demod: fixed audio interpolator. Issue #354
- Implemented interpolation to audio for NFM and SSB demods allowing the use of a 48k audio sink regardless of the channel sample rate
- AM demod: implemented low pass filter after the demod. Issue #352
- Spectrum window: added ability to use the mouse wheel to move the central channel marker line. Issue #343
- Frequency Tracker: make lock converge faster
- Added a splash screen on startup. Issue #343
- SoapySDR output: support floating point type samples. Issue #345
- NFM demod: made high pass audio filter optional to be able to pass DC to other programs via audio. Issue #343
- Added UHD rules in udev (and moved away to sdrangel-docker). Issue #343
- Use Qt precise timer for test and file sources
MIMO support
- "hidden"
--mimooption to activate MIMO functionality (MIMO device set). Makes it optional until it can be officialy released - Updated some base classes to adequately support new MIMO devices