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

etc/codeTemplates/dynamicCode/codedPatchFunction1Template.H

@@ -129,12 +129,6 @@ public:
         //- Return value as a function of (scalar) independent variable
         virtual tmp<Field<${TemplateType}>> value(const scalar x) const;
 
-        //- Is value constant (i.e. independent of x)
-        virtual bool constant() const
-        {
-            return false;
-        }
-
         //- Is value uniform (i.e. independent of coordinate)
         virtual bool uniform() const
         {