ENH: refactored regIOobject searching in object registry

- The findObject() methods are template-typed and used to locate a
  particular Type/name combination.

  Eg,

      volScalarField* ptr = obr.findObject<volScalarField>("xyz");

- The findIOobject() methods are un-typed and use the name only.

  Eg,

      regIOobject* ptr = obr.findIOobject("xyz");

The typed versions will be most commonly used, but the un-typed lookup
can be useful in a templating.

- Simplified findObject* methods to use findIOobject* as the backend.