Add at method to template_numberingt

Previously the only way of getting a value was with operator[], which
makes it too easy to get a non-const reference without meaning to.

This also removes an incompatibility with the previous version of
numbering which was causing a diff between security analyser and CBMC.

Author: NathanJPhillips

src/util/numbering.h

@@ -69,6 +69,11 @@ class template_numberingt final
     return data_.size();
   }
 
+  const key_type &at(size_type t) const
+  {
+    return data_.at(t);
+  }
+
   key_type &operator[](size_type t)
   {
     return data_[t];