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/functionObjects/field/zeroGradient/zeroGradient.C

@@ -176,16 +176,15 @@ bool Foam::functionObjects::zeroGradient::write()
     }
 
     // Consistent output order
-    const wordList outputList = results_.sortedToc();
-    for (const word& fieldName : outputList)
+    for (const word& fieldName : results_.sortedToc())
     {
-        if (foundObject<regIOobject>(fieldName))
-        {
-            const regIOobject& io = lookupObject<regIOobject>(fieldName);
+        const regIOobject* ioptr = findObject<regIOobject>(fieldName);
 
+        if (ioptr)
+        {
             Log << "    " << fieldName << endl;
 
-            io.write();
+            ioptr->write();
         }
     }