Adds airspy USB tuner support and updates the spectral display

component.

-Airspy supported with 10 and 2.5 MSps sample rates.  Additional
decimation may be added later.

-Spectral display updated to include smoothing and filtering options.

Note: the decimated output of a tuner channel from the airspy is
48,076.9 samples per second, versus the normal 48 kHz decimated rate.
This is 1.0015 times the normal rate and doesn't appear to impact the
decoders or audio processing.  Therefore, no fractional resampling has
been added.

src/spectrum/converter/DFTResultsConverter.java

@@ -18,6 +18,12 @@ public abstract class DFTResultsConverter
 	public DFTResultsConverter()
 	{
 	}
+
+	/**
+	 * Specifies the sample size in bits to establish the maximum dynamic range.  
+	 * All FFT bin values will be scaled according to this value.
+	 */
+	public abstract void setSampleSize( double size );
 	
 	public void dispose()
 	{