ENH: simplify objectRegistry access names (issue #322)

New name:  findObject(), cfindObject()
  Old name:  lookupObjectPtr()

      Return a const pointer or nullptr on failure.

  New name:  findObject()
  Old name:  --

      Return a non-const pointer or nullptr on failure.

  New name:  getObjectPtr()
  Old name:  lookupObjectRefPtr()

      Return a non-const pointer or nullptr on failure.
      Can be called on a const object and it will perform a
      const_cast.

- use these updated names and functionality in more places

NB: The older methods names are deprecated, but continue to be defined.

src/engine/engineMesh/fvMotionSolverEngineMesh/fvMotionSolverEngineMesh.C

@@ -91,8 +91,7 @@ void Foam::fvMotionSolverEngineMesh::move()
     if (engineDB_.foundObject<surfaceScalarField>("phi"))
     {
         surfaceScalarField& phi =
-            const_cast<surfaceScalarField&>
-            (engineDB_.lookupObject<surfaceScalarField>("phi"));
+            engineDB_.lookupObjectRef<surfaceScalarField>("phi");
 
         const volScalarField& rho =
             engineDB_.lookupObject<volScalarField>("rho");

src/engine/engineMesh/layeredEngineMesh/layeredEngineMesh.C

@@ -86,8 +86,7 @@ void Foam::layeredEngineMesh::move()
     if (engineDB_.foundObject<surfaceScalarField>("phi"))
     {
         surfaceScalarField& phi =
-            const_cast<surfaceScalarField&>
-            (engineDB_.lookupObject<surfaceScalarField>("phi"));
+            engineDB_.lookupObjectRef<surfaceScalarField>("phi");
 
         const volScalarField& rho =
             engineDB_.lookupObject<volScalarField>("rho");