Updated the Python nineml.user_layer module to match (mostly) the most recent version of the spec (see Specification2.tex and CondensedUL.tex) which itself has some inconsistencies between the text and the appendices.
Added proposed "<array>" tag to complement <scalar>, and fixed some "from_xml()" bugs in the previous commit.
In user_layer.py, added a namespace prefix.
Python user layer classes now inherit from a common base class.
Merge branch 'master' of github.com:INCF/nineml
Merge pull request #2 from apdavison/master

Minor updates to the Python user_layer module.