New update/file format for pubmed bulk xml files

[seandavi]

https://www.nlm.nih.gov/pubs/techbull/mj25/mj25_pubmed_ftp_data.html

Most changes that I see are roughly compatible and I can confirm that the current pubmed_parser works with the new beta-update files. However, there are some changes in format and there is some (nice) additional content. As the format seems to be in flux, I'm adding this issue just for informational purposes, but December 2025 will likely be a switch over to the new format, so there are opportunities to utilize the new format and information by augmenting the parser when those data stabilize.