#2494 P25P1 and NXDN Decoder Enhancements. (#2514)

- Updates P25P1 C4FM decoder to use same pattern developed for NXDN.
- Both share the C4FM symbol processor and equalizer.
- P25P1 updated with Garner style timing error detector
- NXDN filters updated to use a combined RRC and inverse sync filter all in one.
- P25P1 replaces RRC with band limiting filter
- P25P1 enhanced with dynamic PLL balance updates to combat multi-path fading, in addition to the sync detection training window.
- Decoder PPM error reports now only sent after 2x consecutive sync detections to prevent tuner PPM corrections from noisy/isolated false sync detects.
- Soft error correction added to both P25P1 and IDEN.

Co-authored-by: denny <dennis.sheirer@gmail.com>

src/main/java/io/github/dsheirer/dsp/c4fm/C4FMMessageFramer.java

@@ -0,0 +1,34 @@
+package io.github.dsheirer.dsp.c4fm;
+
+import io.github.dsheirer.dsp.symbol.Dibit;
+
+/**
+ * C4FM message framer base class.
+ */
+public abstract class C4FMMessageFramer
+{
+    /**
+     * Process the symbol
+     * @param symbol in radians
+     * @return true if the framer emits a valid message that includes this symbol, so that corrective measurement
+     * can be sent to a feedback decoder.
+     */
+    public abstract boolean process(Dibit symbol);
+
+    /**
+     * Process the symbol decision along with the soft symbol it was decided from.  Framers that can use soft symbols
+     * (ie soft decision decoding) override this method.  Default implementation discards the soft symbol.
+     * @param symbol decision
+     * @param softSymbol equalized soft symbol in radians
+     * @return true if the framer emits a valid message that includes this symbol.
+     */
+    public boolean process(Dibit symbol, float softSymbol)
+    {
+        return process(symbol);
+    }
+
+    /**
+     * A sync pattern is detected
+     */
+    public abstract void syncDetected();
+}