Rename argument: velocity -> difference

src/timestepping.cc

@@ -22,7 +22,7 @@ class TimeSteppingScheme {
                                VectorType &solution) const = 0;
 
   void virtual extractDifference(VectorType const &problem_iterate,
-                                 VectorType &velocity) const = 0;
+                                 VectorType &difference) const = 0;
 
 protected:
   VectorType const ℓ
@@ -76,8 +76,8 @@ class ImplicitEuler
   }
 
   void virtual extractDifference(VectorType const &problem_iterate,
-                                 VectorType &velocity) const {
-    velocity = problem_iterate;
+                                 VectorType &difference) const {
+    difference = problem_iterate;
   }
 };
 
@@ -137,7 +137,7 @@ class ImplicitTwoStep
   }
 
   void virtual extractDifference(VectorType const &problem_iterate,
-                                 VectorType &velocity) const {
-    velocity = problem_iterate;
+                                 VectorType &difference) const {
+    difference = problem_iterate;
   }
 };