This is clever, but for now here I would rather explicitly list the special attributes important to the `FITS_rec` instance by looping over a list like

```
['_convert', '_heapoffset', '_heapsize', '_nfields', '_gap', '_uint', 'names', 'formats']
```

There can still be a special case also for `_coldefs` but maybe put that outside the loop over the other attributes.  Part of the reason for this is that I plan to gradually deprecate this class, and so the less 'magic' it has the better.