ENH: define IOobjectList::csorted(), deprecate some sorted() const methods

- prefer csorted() method for const access since it ensures that the
  return values are also const pointers (for example) even if
  the object itself can be accessed as a non-const.

- the csorted() method already existed for HashTable and
  objectRegistry, but now added to IOobjectList for method name
  consistency (even although the IOobjectList only has a const-access
  version)

ENH: objectRegistry with templated strict lookup

- for lookupClass and csorted/sorted. Allows isType restriction as a
  compile-time specification.

applications/test/gravityMeshObject/Test-gravityMeshObject.C

@@ -57,7 +57,7 @@ int main(int argc, char *argv[])
 
     Info<< "Found: " << objects << nl << endl;
 
-    for (const IOobject& io : objects.sorted<uniformDimensionedVectorField>())
+    for (const IOobject& io : objects.csorted<uniformDimensionedVectorField>())
     {
         if (io.name() == meshObjects::gravity::typeName)
         {