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/sample/ConversionUtils.java

@@ -4,7 +4,6 @@ import java.nio.ByteBuffer;
 import java.nio.ByteOrder;
 import java.nio.ShortBuffer;
 
-import sample.complex.ComplexBuffer;
 import sample.real.RealBuffer;
 
 /**
@@ -57,7 +56,7 @@ public class ConversionUtils
 	 * Converts the float samples in a complex buffer to a little endian 16-bit
 	 * buffer
 	 */
-	public static ByteBuffer convertToSigned16BitSamples( ComplexBuffer buffer )
+	public static ByteBuffer convertToSigned16BitSamples( Buffer buffer )
 	{
 		return convertToSigned16BitSamples( buffer.getSamples() );
 	}