Resolves #373 - Adds support for Tuner frequency error PPM monitoring

and auto-correction for R820T tuners.

-Updated TunerModel and TunerViewPanel to add 2 new columns to
display the current tuner's PPM setting and (when available) the
measured frequency error averaged over 5 second intervals provided by
certain decoders (e.g. P25 C4FM and LSM).  Frequency error value from
P25 decoders is provided from the Costas Loop PLL once 2 or more P25
message sync events have been observed in a short period.
-Updated R820T tuner to auto-correct PPM when measured frequency error
exceeds the +/- 0.4PPM threshold for a period of 30 seconds or more.
Feature can be enabled/disabled by user in the tuner editor panel.

Resolved several issues with P25 C4FM/LSM decoders that was impacting
performance related to channel misalignment from tuner PPM offsets.

src/main/java/io/github/dsheirer/spectrum/WaterfallPanel.java

@@ -455,7 +455,10 @@ public class WaterfallPanel extends JPanel implements DFTResultsListener,
             @Override
             public void run()
             {
-                mMemoryImageSource.newPixels(mPixels, mColorModel, 0, mDFTSize);
+                if(mMemoryImageSource != null)
+                {
+                    mMemoryImageSource.newPixels(mPixels, mColorModel, 0, mDFTSize);
+                }
             }
         });
     }