(nearing the end) Update to learning map domain elements

[Robert D Anderson <robander@us.ibm.com>]

There are now 11 elements in the Learning Map Domain, such as learningContentRef, learningGroup, and similar. About half of these declare a default for @type, and the others do not. In some cases the spec shows a default, but the DTD/XSD does not implement a default. In one case, the spec lists a series of values as the "default", but these are really just a description of expected values (DTD/XSD does not give a default here):
http://docs.oasis-open.org/dita/v1.2/os/spec/langref/learningContentRef.html

I've talked with John Hunt about this, and he said if he had this to do over, he wouldn't give defaults for @type for any of these. Removing the default will simplify the specification, and is backwards compatible, so I'm going to proceed with this unless the TC objects. I'll also add it to the list of "code fixes" for DTD/XSD, so that we can remove defaults from the implementation once that work begins.

Thanks,

Robert D Anderson
IBM Authoring Tools Development
Chief Architect, DITA Open Toolkit (http://dita-ot.sourceforge.net/)