class attribute for tunit element

I declared the tunits layer in the dtd with an extra class attribute (for the element name)
```
<!ELEMENT tunits (#PCDATA|tunit)*>

<!ATTLIST tunits
  id CDATA #IMPLIED>

<!ELEMENT tunit EMPTY>

<!ATTLIST tunit
  id CDATA #REQUIRED
  offset CDATA #REQUIRED
  length CDATA #REQUIRED
  class CDATA #IMPLIED>
```