ENH: cleanup autoPtr class (issue #639)

Improve alignment of its behaviour with std::unique_ptr

  - element_type typedef
  - release() method - identical to ptr() method
  - get() method to get the pointer without checking and without releasing it.
  - operator*() for dereferencing

Method name changes

  - renamed rawPtr() to get()
  - renamed rawRef() to ref(), removed unused const version.

Removed methods/operators

  - assignment from a raw pointer was deleted (was rarely used).
    Can be convenient, but uncontrolled and potentially unsafe.
    Do allow assignment from a literal nullptr though, since this
    can never leak (and also corresponds to the unique_ptr API).

Additional methods

  - clone() method: forwards to the clone() method of the underlying
    data object with argument forwarding.

  - reset(autoPtr&&) as an alternative to operator=(autoPtr&&)

STYLE: avoid implicit conversion from autoPtr to object type in many places

- existing implementation has the following:

     operator const T&() const { return operator*(); }

  which means that the following code works:

       autoPtr<mapPolyMesh> map = ...;
       updateMesh(*map);    // OK: explicit dereferencing
       updateMesh(map());   // OK: explicit dereferencing
       updateMesh(map);     // OK: implicit dereferencing

  for clarity it may preferable to avoid the implicit dereferencing

- prefer operator* to operator() when deferenced a return value
  so it is clearer that a pointer is involve and not a function call
  etc    Eg,   return *meshPtr_;  vs.  return meshPtr_();

applications/utilities/mesh/manipulation/setSet/setSet.C

@@ -472,7 +472,7 @@ bool doCommand
                         setSource().applyToSet(topoSetSource::NEW, currentSet);
 
                         // Combine new value of currentSet with old one.
-                        currentSet.subset(oldSet);
+                        currentSet.subset(oldSet());
                     }
                     break;
                 }
@@ -814,7 +814,7 @@ int main(int argc, char *argv[])
         // Main command read & execute loop
         // ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-        autoPtr<IFstream> fileStreamPtr(nullptr);
+        autoPtr<IFstream> fileStreamPtr;
 
         if (batch)
         {