STYLE: prefer isHeaderClass<Type>() check instead of Type::typeName

- use typeHeaderOk<regIOobject>(false) for some generic file existence
  checks. Often had something like labelIOField as a placeholder, but
  that may be construed to have a particular something.

applications/utilities/preProcessing/setExprFields/readFields.H

@@ -121,15 +121,14 @@ class readFieldsHandler
                 (
                     fieldName,
                     mesh_.time().timeName(),
-                    mesh_,
+                    mesh_.thisDb(),
                     IOobject::MUST_READ,
                     IOobject::NO_WRITE
                 );
 
                 const bool ok =
                 (
-                    io.typeHeaderOk<regIOobject>(false) // Preload header info
-                 && io.hasHeaderClass()                 // Extra safety
+                    io.typeHeaderOk<regIOobject>(false)
                  &&
                     (
                         loadField<scalar>(io)

applications/utilities/preProcessing/setExprFields/setExprFields.C

@@ -270,9 +270,8 @@ void evaluate
             IOobject::MUST_READ,
             IOobject::NO_WRITE
         );
-        io.typeHeaderOk<IOobject>(false);
 
-        if (!io.hasHeaderClass() || io.isHeaderClass<IOobject>())
+        if (!io.typeHeaderOk<regIOobject>(false))
         {
             FatalErrorInFunction
                 << "Field '" << fieldName