HashTbl changes

- iterators store pointers instead of references to the HashTbl.
  This lets us use the default bitwise copy/assignment

- add empty constructor for iterators. It returns the equivalent to end().
  This lets us do this:
      HashTbl<label>::iterator iter;
      // some time later
      iter = find(Value);

- erase(const HashTbl<AnyType, Key, AnyHash>&) is now more generous.
  Only the Key type matters, not the hashing function.

applications/test/HashTable/hashTableTest.C

@@ -40,6 +40,7 @@ using namespace Foam;
 int main()
 {
     HASHTABLE_CLASS<double> table1(13);
+    HASHTABLE_CLASS<double>::iterator iter;
 
     table1.insert("aaa", 1.0);
     table1.insert("aba", 2.0);
@@ -52,8 +53,12 @@ int main()
     table1.insert("adx", 9.0);
     table1.insert("aec", 10.0);
 
+    // erase by key
     table1.erase("aaw");
-    table1.erase("abs");
+
+    // erase by iterator
+    iter = table1.find("abs");
+    table1.erase(iter);
 
     Info<< "\ntable1 toc: " << table1.toc() << endl;
     Info<< "\ntable1 sortedToc: " << table1.sortedToc() << endl;