2 requested fixes in DTDs

Erik Hennum sent me notifications about the following two items that should
be changed in the classification domain. These came up while Eric Sirois
was doing additional Schema testing.

1. Eric noticed that the current definition of subjectCell violates the
Unique Particle Attribution laws of XML Schema. Specifically, it allows
topicsubject twice, initially as a direct child, and again because it is a
domain substitution element for topicref. The fix for this is to remove
topicsubject as a direct child. This fix does not impact any existing
documents, and the same content model is possible, it just makes it legal.

Testing of this led to the second discovery:

2. The