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/dynamicFvMesh/dynamicMultiMotionSolverFvMesh/dynamicMultiMotionSolverFvMesh.C

@@ -182,8 +182,7 @@ bool Foam::dynamicMultiMotionSolverFvMesh::update()
 
     if (foundObject<volVectorField>("U"))
     {
-        const_cast<volVectorField&>(lookupObject<volVectorField>("U"))
-            .correctBoundaryConditions();
+        lookupObjectRef<volVectorField>("U").correctBoundaryConditions();
     }
     else if (!hasWarned)
     {