Changes <file ID="PDF00001"> to <file ID="PDF_ISSUELEVEL"> throughout.

As per Michael Dewsnip's memo, the ID attribute of <file> elements
referencing the issue-level pdf for an issue MUST be PDF_ISSUELEVEL in
order for Veridian to find it.  This patch changes all METS in the tree
that still have PDF00001 as the value of that ID.