Add the operator that had been in use by Plugin and PluginManager.

src/org/nwapw/abacus/function/Operator.java

@@ -0,0 +1,27 @@
+package org.nwapw.abacus.function;
+
+public abstract class Operator {
+
+    private OperatorAssociativity associativity;
+    private int precedence;
+    private Function function;
+
+    public Operator(OperatorAssociativity associativity, int precedence, Function function){
+        this.associativity = associativity;
+        this.precedence = precedence;
+        this.function = function;
+    }
+
+    public OperatorAssociativity getAssociativity() {
+        return associativity;
+    }
+
+    public int getPrecedence() {
+        return precedence;
+    }
+
+    public Function getFunction() {
+        return function;
+    }
+
+}