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).

src/functionObjects/field/zeroGradient/zeroGradient.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2016-2018 OpenCFD Ltd.
+    Copyright (C) 2016-2020 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -119,7 +119,7 @@ bool Foam::functionObjects::zeroGradient::read(const dictionary& dict)
 
     Info<< type() << " fields: " << selectFields_ << nl;
 
-    resultName_ = dict.lookupOrDefault<word>("result", type() + "(@@)");
+    resultName_ = dict.getOrDefault<word>("result", type() + "(@@)");
 
     // Require '@@' token for result, unless a single (non-regex) source field
     return