- in some circumstances we need to pass a bool value upwards to the
  caller and know if the true/false value was set based on real input
  or is a default value.

  Eg, in the object::read() we might normally have

     enabled_(dict.readIfPresent(key, true));

  but would lose information about why the value is true/false.

  We can change that by using

     enabled_(dict.readIfPresent<Switch>(key, Switch::DEFAULT_ON));

  After which we can use this information is testing.

      if
      (
          child.enabled().nonDefault()
        ? child.enabled()
        : parent.enabled()
      )
      { ... }

   And thus enable output if the parent requested it explicitly or by
   default and it has not been explicitly disabled in the child.

  No difference when testing as a bool and the text representation
  of DEFAULT_ON / DEFAULT_OFF will simply be "true" / "false".

ENH: add construction of Switch from dictionary (similar to Enum)