ENH: unify use of dictionary method names

- previously introduced `getOrDefault` as a dictionary _get_ method,
  now complete the transition and use it everywhere instead of
  `lookupOrDefault`. This avoids mixed usage of the two methods that
  are identical in behaviour, makes for shorter names, and promotes
  the distinction between "lookup" access (ie, return a token stream,
  locate and return an entry) and "get" access (ie, the above with
  conversion to concrete types such as scalar, label etc).

applications/utilities/mesh/generation/foamyMesh/cellSizeAndAlignmentGrid/cellSizeAndAlignmentGrid.C

@@ -6,6 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2012-2016 OpenFOAM Foundation
+    Copyright (C) 2020 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -310,7 +311,7 @@ int main(int argc, char *argv[])
             IOobject::NO_WRITE
         ),
         foamyHexMeshDict.subDict("geometry"),
-        foamyHexMeshDict.lookupOrDefault("singleRegionName", true)
+        foamyHexMeshDict.getOrDefault("singleRegionName", true)
     );
 
     conformationSurfaces geometryToConformTo