Scope update and Test Sink plugin
- New device sink plugin for Tx testing: https://github.com/f4exb/sdrangel/tree/master/plugins/samplesink/testsink
- Channel Analyzer: set trace length to FFT length when auto-correlation by IFFT is used
Other changes and fixes
- Tx plugins: corrected FIFO begin iterator vs number of samples read
- GLSpectrum: set default window to Hanning
- More preparatory works for MIMO channels support