Update usage docs

[cole]

Adds basic usage documentation. I don't think this covers all of @replaceafill's use cases yet, but it's a start hopefully.

Connects to archivematica/Issues#609.

[replaceafill]

It's definitely a nice start @cole.

Regarding use cases, these are some things we've been doing with the library to parse the METS:

• Iterate file entries in the <fileSec> according to their @USE:

mets = metsrw.METSDocument.fromfile(path)
original_files = [entry for entry in mets.all_files() if entry.use == "original"]

• Filter PREMIS events of a file entry by type:

# entry is a FSEntry object
reingestion_events = [
    event for event in entry.get_premis_events() if event.type == 'reingestion'
]

• Use the METS tree to get the <amdSec> from a file entry:

# entry is a FSEntry object
amdSec = mets.tree.find(
    'mets:amdSec[@ID="{}"]'.format(entry.admids[0]),
    namespaces={...},
)

[cole]

Thanks @replaceafill! I'm going to merge this and try to address those cases in a new PR.

[currmie]

Hi @cole - just some more food for thought and ideas for the future. What you've got has been super helpful!

METS provides a way to link digital content with applications or code in the behaviorSec that can be used with other info in a METS document to transform or display the digital object(s). Not sure if this section is used much by the AtoM community. Just thought it might be nice to include the behaviorSec elements and attributes in the metsrw API Documentation > Metadata classes section.