add validation method to check ALTO XML zipfile

[rlskoeser]

• check that contents are xml files
• check that xml files are ALTO XML

[tanhaow]

The following changes are needed to make:

• non-xml files are ignored (but logged - maybe info)
• non-alto xml files are ignored (but log a warning)
• error if no alto xml files are found in the zipfile

Also, we won't need to iterate over the files twice, the parsing should also handle the validation piece you're doing now - i.e., if we attempt to open a non-alto file as our alto xml object, then the object should raise an exception that we can catch and report here in the input class.