ENH: additional variants of IOobjectList findObject()

- cfindObject() for const pointer access.

- getObject() for mutable non-const pointer access, similar to the
     objectRegistry::getObjectPtr()

- cfindObject(), findObject(), getObject() with template type access
  to also check the headerClassName.

  For example,

      cfindObject("U")  ->  good
      cfindObject<volVectorField>("U") -> good
      cfindObject<volScalarField>("U") -> nullptr

  This allows inversion of looping logic.

    1) Obtain the names for a particular Type

       for (const word& objName : objs.sortedNames<Type>())
       {
           const IOobject* io = objs[objName];
           ...
       }

    2) Use previously obtained names and apply to a particular Type

       for (const word& objName : someListOfNames)
       {
           const IOobject* io = objs.cfindObject<Type>(objName);
           if (io)
           {
               ...
           }
       }

applications/test/IOobjectList/Test-IOobjectList.C

@@ -52,7 +52,7 @@ void reportDetail(const IOobjectList& objects)
     for (const word& key : objects.sortedNames())
     {
         // Canonical method name (NOV-2018)
-        IOobject* io = objects.findObject(key);
+        const IOobject* io = objects.findObject(key);
 
         label count = 0;
 
@@ -82,6 +82,55 @@ void reportDetail(const IOobjectList& objects)
 }
 
 
+void printFound(const IOobject* ptr)
+{
+    Info<< (ptr ? "found" : "not found") << nl;
+}
+
+
+void findObjectTest(const IOobjectList& objs)
+{
+    Info<< "Test findObject()" << nl << nl;
+
+    const int oldDebug = IOobject::debug;
+    IOobject::debug = 1;
+
+    {
+        Info<< "cfindObject(U)" << nl;
+        const IOobject* io = objs.cfindObject("U");
+        printFound(io);
+    }
+
+    {
+        Info<< "getObject(U)" << nl;
+        IOobject* io = objs.getObject("U");
+        printFound(io);
+    }
+
+    {
+        Info<< "cfindObject<void>(U)" << nl;
+        const IOobject* io = objs.cfindObject<void>("U");
+        printFound(io);
+    }
+
+    {
+        Info<< "cfindObject<volScalarField>(U)" << nl;
+        const IOobject* io = objs.cfindObject<volScalarField>("U");
+        printFound(io);
+    }
+
+    {
+        Info<< "cfindObject<volVectorField>(U)" << nl;
+        const IOobject* io = objs.cfindObject<volVectorField>("U");
+        printFound(io);
+    }
+
+    Info<< nl;
+
+    IOobject::debug = oldDebug;
+}
+
+
 template<class Type>
 void filterTest(const IOobjectList& objs, const wordRe& re)
 {
@@ -219,6 +268,8 @@ int main(int argc, char *argv[])
 
         report(objects);
 
+        findObjectTest(objects);
+
         classes.filterKeys(subsetTypes);
         Info<<"only retain: " << flatOutput(subsetTypes) << nl;
         Info<<"Pruned: " << classes << nl;