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/source/tuner/TunerController.java

@@ -41,6 +41,7 @@ public abstract class TunerController implements Tunable, ISourceEventProcessor,
     private int mMiddleUnusableHalfBandwidth;
     private double mUsableBandwidthPercentage;
     private Listener<SourceEvent> mSourceEventListener;
+    private int mMeasuredFrequencyError;
 
     /**
      * Abstract tuner controller class.  The tuner controller manages frequency bandwidth and currently tuned channels
@@ -242,6 +243,46 @@ public abstract class TunerController implements Tunable, ISourceEventProcessor,
         return (int)(getUsableBandwidth() / 2);
     }
 
+    /**
+     * Current measured frequency error as received/reported from certain downstream decoders.
+      * @return measured frequency error in hertz averaged over 1-second intervals
+     */
+    public int getMeasuredFrequencyError()
+    {
+        return mMeasuredFrequencyError;
+    }
+
+    /**
+     * Indicates if this tuner controller has a non-zero measured frequency error value.
+     */
+    public boolean hasMeasuredFrequencyError()
+    {
+        return mMeasuredFrequencyError != 0;
+    }
+
+    /**
+     * Calculates the current PPM error value as measured by certain downstream decoders.
+     * @return
+     */
+    public double getPPMFrequencyError()
+    {
+        if(hasMeasuredFrequencyError())
+        {
+            return mMeasuredFrequencyError / ((double)getFrequency() / 1E6d);
+        }
+
+        return 0.0d;
+    }
+
+    /**
+     * Sets the measured frequency error average.
+     * @param measuredFrequencyError in hertz averaged over a 5 second interval.
+     */
+    public void setMeasuredFrequencyError(int measuredFrequencyError)
+    {
+        mMeasuredFrequencyError = measuredFrequencyError;
+    }
+
     /**
      * Sets the listener to be notified any time that the tuner changes frequency
      * or bandwidth/sample rate.