STYLE: avoid deprecation warnings for autoPtr set() method

- set() was silently deprecated in favour of reset() FEB-2018
  since the original additional check for overwriting an existing
  pointer was never used. The reset(...) name is more consistent
  with unique_ptr, tmp etc.

  Now emit deprecations for set().

- use direct test for autoPtr, tmp instead of valid() method.
  More consistent with unique_ptr etc.

STYLE: eliminate redundant ptr() use on cloned quantities

applications/utilities/postProcessing/dataConversion/foamToVTK/writePointFields.H

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2018-2020 OpenCFD Ltd.
+    Copyright (C) 2018-2022 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -57,7 +57,7 @@ bool writePointField
     const fvMeshSubsetProxy& proxy
 )
 {
-    if (!tfield.valid())
+    if (!tfield)
     {
         return false;
     }
@@ -73,7 +73,7 @@ bool writePointField
         tproxied = tfield;
     }
 
-    if (!tproxied.valid())
+    if (!tproxied)
     {
         // Or Error?
         return false;