Move ExpressionNodes to their own directory.

VAR.ExpressionEvaluator/ExpressionNodes/ExpressionNumberNegateNode.cs

@@ -0,0 +1,30 @@
+using System;
+
+namespace VAR.ExpressionEvaluator
+{
+    public class ExpressionNumberNegateNode : ExpressionUnaryNode
+    {
+        public ExpressionNumberNegateNode(IExpressionNode node) :
+            base(node, NumberNegateOp)
+        {
+        }
+
+        private static object NumberNegateOp(object value)
+        {
+            if (value is string)
+            {
+                if (decimal.TryParse((string)value, out decimal dec) == false)
+                {
+                    throw new Exception(string.Format("Can't convert to decimal string value \"{0}\"", (string)value));
+                }
+                value = dec;
+            }
+
+            if ((value is decimal) == false)
+            {
+                throw new Exception("Can't negate non decimal values");
+            }
+            return -(decimal)value;
+        }
+    }
+}