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

src/functionObjects/field/histogram/histogram.C

@@ -160,9 +160,9 @@ bool Foam::functionObjects::histogram::write()
 
     const volScalarField& field =
     (
-         fieldPtr.valid()
-       ? fieldPtr()
-       : obr_.lookupObject<volScalarField>(fieldName_)
+        fieldPtr
+      ? fieldPtr()
+      : obr_.lookupObject<volScalarField>(fieldName_)
     );