Feature object registry search

  - objectRegistry search, erase methods
  - clip, minMax
  - function object triggering
  ...

- 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.

- similar to what erase() does, but as a mutable operation (#1180)

- replace basicThermo lookupAndCheckout (commit 25c74188) with
  the new objectRegistry::checkOut() method.