Changes made during port

The steps taken during the port were:

template.tex is based on original/agujournaltemplate.tex started with a copy-paste
leaving all original comments in place
modified initial package loading
- removed url as it is in base packages
Added options.journal_name as aconditional with fallback to Geophysical Research Letters
inserting Title
Added authors with optional affiliations
Ported corresponding author, connecting this to an option
Ported keypoints taking these from parts content section and assuming an array, falling back to placeholder content
Ported abstract as parts
Ported plain_language_summary as parts
Inserted CONTENT marker, note there was no example content inline here
Ported acknowledgments as parts
set bibliography to main.bib

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