330 WIP on redesigning the P25 C4FM decoder. Creates a new decoder

based on the P25 LSM decoder using a costas loop and a gardner detector.
Updates the Demodulator Viewer FX application to add in a symbol chart
and a phase error chart.  The decoder won't lock on yet ... still a wip.

src/main/java/io/github/dsheirer/dsp/psk/IQPSKSymbolDecoder.java

@@ -0,0 +1,29 @@
+/*******************************************************************************
+ * sdr-trunk
+ * Copyright (C) 2014-2018 Dennis Sheirer
+ *
+ * This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public
+ * License as published by  the Free Software Foundation, either version 3 of the License, or  (at your option) any
+ * later version.
+ *
+ * This program is distributed in the hope that it will be useful,  but WITHOUT ANY WARRANTY; without even the implied
+ * warranty of  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License  along with this program.
+ * If not, see <http://www.gnu.org/licenses/>
+ *
+ ******************************************************************************/
+package io.github.dsheirer.dsp.psk;
+
+import io.github.dsheirer.dsp.symbol.Dibit;
+import io.github.dsheirer.sample.complex.Complex;
+
+public interface IQPSKSymbolDecoder
+{
+    /**
+     * Decodes the symbol and converts it into a Dibit
+     * @param symbol to decode
+     * @return decoded dibit
+     */
+    Dibit decode(Complex symbol);
+}