Generate METS file along with ALTO

In order to link images with ALTO payload (and instead of abusing ALTO syntax that would lead nowhere, see #72 (closed)), we should build a simple METS file along with all ALTO files.

A new CLI option --mets will enable this generation (disabled by default for now).

⚠ there is no image download here, we'll just use IIIF urls.

When used, a METS file used for file linking (no metadata) is generated with these sections:

• mets:fileSec describes all our files:
  • generated ALTO files, in a mets:fileGrp
  • remote images using IIIF urls, in a mets:fileGrp
• mets:structMap links file pointers (so our ALTO file and our image declaration);

Here is a good example of such structure https://memory.loc.gov/diglib/ihas/loc.natlib.ihas.200033407/mets.xml

The generated file is simply named mets_entrypoint.xml