Utilties for working with CEX data: the CexUtils module

CEX is a plain-text format for organizing citable data. As a file format, CEX does not specify what kinds of URN values may be used to identify scholarly resources. The generically applicable Cite2Urn can be used to identify the contents of citable collections and sets of relations among citable resources. The CexUtils submodule provides utilities for filtering and extracting content from CEX sources identified by Cite2Urns.