Add fabs(adouble) as a synonym for abs(adouble)

dune/fufem/utilities/adolcnamespaceinjections.hh

@@ -61,6 +61,10 @@ namespace std
      return abs_hack(a);
    }
 
+   adouble fabs(adouble a) {
+     return abs_hack(a);
+   }
+
    adouble log(adouble a) {
      return log_hack(a);
    }