Fix file splitting on sequencelinkout files

Reworked generation of sequencelinkoutXXXX.xml files to effectively limit file size.

Limits are approximate due to the XML tags before and after an `objectId` element in the `objectList`, but this is still controlled and can generate files within the size contstraints. Previously the file writer would not split within an objectList, and a single objectList was over 32MB.

Tested by setting object limit to 10 files on my local machine, generating 717 files, then validating them with `find . -name "sequencelinkout*.xml" -exec xmllint --valid {} --noout \;`. Found a good deal of edge cases and corrected them.

Fixes #8