Add utility script to extract CONTENTdm pointer from MODS.xml

[mjordan]

We are developing utility scripts (like extras/scripts/check_files.php) that write out the path to problematic newspaper issues in MIK output. In order to reprocess these problematic newspaper issues, it would be useful to be able to convert the issue paths (which for SFU use the publication date as the directory name) to a list of corresponding (issue-level) CONTENTdm pointers.

@MarcusBarnes suggested pulling the pointers from the CONTENTdm data embedded in the issue-level MODS.xml files.