Read and parse EML docs incrementally for faster rendering

[robyngit]

MetacatUI downloads entire EML documents before parsing them in the Editor. This can be slow for large documents, especially those with many entities. In the viewer, we don't download or parse EML docs but instead rely on getting metadata from the view service, presumably to speed up rendering. This makes customizing the apperance of the landing page challenging.

Since the ReadableStream interface is now supported in all major browsers, we can use it to read EML in chunks and parse docs incrementally as they're being downloaded. This would speed things up and reduce memory usage, allowing users to view large EML docs without waiting for the entire document to download, and allowing us to build a more customizable and modern landing page.

To implement this, we would:

• Fetch the document using the fetch() API
• Get a ReadableStream for the document body
• Use a streaming XML parser, like sax-js, to handle processing partial XML data

For this to be possible on the front-end, Metacat would need to support range requests for EML documents.