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