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/SourceEvent.java

@@ -31,6 +31,8 @@ public class SourceEvent
         NOTIFICATION_FREQUENCY_AND_SAMPLE_RATE_UNLOCKED,
         NOTIFICATION_FREQUENCY_CHANGE,
         NOTIFICATION_FREQUENCY_CORRECTION_CHANGE,
+        NOTIFICATION_MEASURED_FREQUENCY_ERROR,
+        NOTIFICATION_MEASURED_FREQUENCY_ERROR_SYNC_LOCKED,
         NOTIFICATION_SAMPLE_RATE_CHANGE,
         NOTIFICATION_STOP_SAMPLE_STREAM,
 
@@ -124,6 +126,14 @@ public class SourceEvent
         return mSource;
     }
 
+    /**
+     * Sets the source associated with this event
+     */
+    public void setSource(Source source)
+    {
+        mSource = source;
+    }
+
     /**
      * Indicates if this event contains an optional source
      */
@@ -189,6 +199,28 @@ public class SourceEvent
         return new SourceEvent(Event.NOTIFICATION_CHANNEL_FREQUENCY_CORRECTION_CHANGE, frequencyCorrection);
     }
 
+    /**
+     * Creates a new frequency error measurement notification event.
+     *
+     * @param frequencyError in hertz
+     */
+    public static SourceEvent frequencyErrorMeasurement(long frequencyError)
+    {
+        return new SourceEvent(Event.NOTIFICATION_MEASURED_FREQUENCY_ERROR, frequencyError);
+    }
+
+    /**
+     * Creates a new frequency error measurement notification event.  This event is different from
+     * the raw frequency error measurement and indicates that the current state of the PLL is
+     * locked and tracking the signal.
+     *
+     * @param frequencyError in hertz
+     */
+    public static SourceEvent frequencyErrorMeasurementSyncLocked(long frequencyError, String eventDescription)
+    {
+        return new SourceEvent(Event.NOTIFICATION_MEASURED_FREQUENCY_ERROR_SYNC_LOCKED, frequencyError, eventDescription);
+    }
+
     /**
      * Creates a new sample rate change event
      *
@@ -288,7 +320,7 @@ public class SourceEvent
     {
         return "SOURCE EVENT:" + mEvent +
                " VALUE:" + (mValue != null ? mValue : "(empty)") +
-               " SOURCE:" + (mSource != null ? mSource.getClass() : "(null)") +
+               " SOURCE:" + (mSource != null ? mSource.toString() : "(null)") +
                " DESCRIPTION:" + (mEventDescription != null ? mEventDescription : "");
     }
 }