[Mat/MPL] Readability: Rename 'u'->'displacement'.

MaterialLib/MPL/VariableType.cpp

@@ -41,9 +41,9 @@ Variable convertStringToVariable(std::string const& input)
     {
         return Variable::liquid_saturation;
     }
-    if (boost::iequals(input, "u"))
+    if (boost::iequals(input, "displacement"))
     {
-        return Variable::u;
+        return Variable::displacement;
     }
 
     OGS_FATAL(

MaterialLib/MPL/VariableType.h

@@ -45,7 +45,7 @@ enum Variable : int
     liquid_density,
     temperature,
     liquid_saturation,
-    u,
+    displacement,
     number_of_variables
 };