Add new key modifier fixed, default symbol '='

The new modifer is used to declare a parameter to be fixed and not
changeable in subsequent classes. If ignore_merging_onto_fixed is
False then trying to change a fixed parameter will generate an error
if ignore_merging_onto_fixed is True the the parameter will not be
changed and no error will be generated
diff --git a/reclass/values/value.py b/reclass/values/value.py
index 286407c..ffb116f 100644
--- a/reclass/values/value.py
+++ b/reclass/values/value.py
@@ -24,6 +24,7 @@
         self._settings = settings
         self._uri = uri
         self._overwrite = False
+        self._fixed = False
         if isinstance(value, string_types):
             if parse_string:
                 try:
@@ -49,6 +50,14 @@
         self._overwrite = overwrite
 
     @property
+    def fixed(self):
+        return self._fixed
+
+    @fixed.setter
+    def fixed(self, fixed):
+        self._fixed = fixed
+
+    @property
     def uri(self):
         return self._uri