Opl update update

The main changes are to use cross-listing information for the OPL.  Some sections will now appear in more than one place.  This allows all say vector geometry problems to be in vector geometry, but also in multivariable calculus (a multivariable calculus instructor may not think to look in vector geometry).  The actual cross-listing is done in the OPL repository (the file Taxonomy2), so after this, additional cross-lists will not need changes to webwork2.  While this is a new feature, it is worth getting in sooner rather than later since it may seriously affect usability of the OPL (affects all versions of library browser and the homework manager).

We also prune from the DBsubject/chapter/section menus items which have no problems.

There are also minor bug fixes: matching of DBsubject/chapter/section names should be case-sensitive, and MLT entries should have a default value.