Split library into submodules and refactored 

The main changes in these commits are:

* Split library.xq into submodules that get imported into the same namespace
* Added methods to identify problems, procedures, meds, etc that take the CCDA root element and a sequence of items containing the codes to look for
* Added library modules to contain variable definitions for sequences of items identifying problems, procedures, meds, etc 

I thought that since this is a somewhat substantial change that I wouild not merge the pull request myself. Instead I would like for someone else to do a quick code review and provide feedback if necessary before merging.