ENH: add get() retrieval of a pointer from PtrLists, HashPtrTable

- naming similarity with autoPtr, unique_ptr and other containers.

  For UPtrList derivatives, this is equivalent to the existing
  operator(). The read-only variant is also equivalent to the
  single-parameter 'set(label)' method.

  With PtrList<T> list(...) :

      const T* ptr = list.get(10);
      if (ptr)
      {
          ptr->method();
      }

  vs.
      if (list.set(10))
      {
          list[10].method();
      }

  For HashPtrTable there is only a read-only variant which is equivalent
  to testing for existence and for value.

  With HashPtrTable<T> hash(...) :

      const T* ptr = list.get("key");
      if (ptr)
      {
          ptr->method();
      }

  vs.
      if (list.found("key"))
      {
          // Fails on null pointer!!
          list["key"].method();
      }

Use of get() is largely a matter of taste or local coding requirements

applications/test/HashPtrTable/Test-HashPtrTable.C

@@ -198,6 +198,24 @@ int main()
 
         Info<< "Table: " << tbl << nl;
 
+        Info<< nl << "Check exists, non-null" << nl;
+
+        for (const word& k : { "abc", "foo", "pi" })
+        {
+            Info<< "    " << k << ' ';
+
+            const auto* inspect = tbl.get(k);
+
+            if (inspect)
+            {
+                Info<< *inspect << nl;
+            }
+            else
+            {
+                Info<< "(null)" << nl;
+            }
+        }
+
         Info<< nl << "... overwrite again" << nl;
 
         tbl.set("abc", new Scalar(42.1));