[osis-core] Change to <work> type and subType

[Todd Tillinghast]

Patrick and I talked earlier today about the type and subType attributes
of <work>.

PROPOSAL: Remove "content" or "declaration" and put the list currently
on subType on type.  

The reasoning being that you know "content" or "declaration" based on
the other value.  The reasoning Patrick and Steve had when including the
"content"/"declaration" attribute was that people would not know which
the other value was.  The same reasoning would apply to those encoding.
The difference is that the encoder would introduce error.  

Until I read Patrick's post earlier today I had used "content" and
"declaration" differently that it was intended.  By removing
"content"/"declaration", we reduce confusion and eliminate the potential
for error.

Todd