ENH: add Function1 constant() member

- marks if the value is considered to be independent of 'x'.
  Propagate into PatchFunction1 instead ad hoc checks there.

- adjust method name in PatchFunction1 to 'whichDb()' to reflect
  final changes in Function1 method names.

ENH: add a Function1 'none' placeholder function

- This is principally useful for interfaces that expect a Function1
  but where it is not necessarily used by a particular submodel.

TUT: update Function1 creation to use objectRegistry

tutorials/multiphase/MPPICInterFoam/twoPhasePachuka/0.orig/U

@@ -68,7 +68,10 @@ boundaryField
 
                 if (!baseVel)
                 {
-                    baseVel = Function1<scalar>::New("timeFunction", dict);
+                    baseVel = Function1<scalar>::New
+                    (
+                        "timeFunction", dict, &db()
+                    );
                 }
 
                 const bool verbose = dict.getOrDefault<bool>("verbose", false);
@@ -76,7 +79,10 @@ boundaryField
                 if (!baseDir && dict.found("directionFunction"))
                 {
                     // ie, NewIfPresent
-                    baseDir = Function1<vector>::New("directionFunction", dict);
+                    baseDir = Function1<vector>::New
+                    (
+                        "directionFunction", dict, &db()
+                    );
 
                     InfoErr
                         << "Function1 for direction" << nl;