catalog_pending_import process needs to create MODS from source_urn

[balmas]

The URN reservation form now populates a field in the versions table, source_urn, that contains the urn of the preceding version (used in the case when a new version is created from an old, such as in the P4-to-P5 conversions). (See #6 )

The catalog_pending_import process should check this field, and create a new MODS record for the new version, based upon the old version's MODS record if it's available. Essentially, it should automate the process documented here: https://github.com/PerseusDL/tei-conversion-tools/wiki/Assigning-and-Updating-URNs#add-mods-file-for-new-urn-to-catalog_pending

[balmas]

This is what the import process should do:

1. Find the MODS file for the original version in the https://github.com/PerseusDL/catalog_data repository (these are kept in a directory structure organized by textgroup and work so they should be easy to find)
2. copy this file, named according to the new version into the directory for the author in the https://github.com/PerseusDL/catalog_pending directory, it for the new version (naming convention is textgroup.work.version.mods.xml e.g. tlg0008.tlg001.perseus-grc2.mods.xml)
3. make sure the schema referenced in the parent mods:mods element is version 3.6, and update it to that if not: <mods:mods xmlns="http://www.loc.gov/mods/v3" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:mods="http://www.loc.gov/mods/v3" xsi:schemaLocation="http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-6.xsd">
4. change the mods:identifier element with the old URN into a mods:relatedItem @type='preceding' element e.g change <mods:identifier type="ctsurn">urn:cts:greekLit:tlg0008.tlg001.perseus-grc1</mods:identifier> into <relatedItem type="preceding" otherTypeURI="tlg0008.tlg001.perseus-grc1"/>
5. add a new mods:identifier @type="ctsurn" element for the new urn. E.g.
   <mods:identifier type="ctsurn">urn:cts:greekLit:tlg0008.tlg001.perseus-grc2</mods:identifier>
6. add a link to the GitHub repo for the new URN to the MODS file??
7. commit and push the new file to the catalog_pending repository