Separate templates for child/page-level MODS files from code, and make their locations configurable

[mjordan]

The MODS.xml files generated by MIK as described in the following toolchain guides use templates that exist within the writer class files. We should separate out these templates into their own files and allow users to configure the path to those template files.

https://github.com/MarcusBarnes/mik/wiki/Toolchain:-CONTENTdm-newspapers https://github.com/MarcusBarnes/mik/wiki/Toolchain:-CSV-newspapers https://github.com/MarcusBarnes/mik/wiki/Toolchain:-CSV-compound-objects

We could provide default templates in a templates directory under src/, for example, and within the [WRITER] section of each .ini, provide a child_metadata_template_path or similar option.

We'd need to document which variables are passed to the templates.

[mjordan]

Perhaps we can use the new InsertXmlFromTemplate metadata manipulator, or a specialized version of it, to do this?