Allow namespace prefixes other than 'None' in PageXML

Eynollah, e.g., produces PageXML files that use an explicit prefix (xmlns:pc="http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15").

calamari_ocr/ocr/dataset/datareader/pagexml/reader.py, however, expects the prefix to be ‘None’ and throws an error when processing an eynollah pagexml.

When I change line 120 of reader.py from

ns = {"ns": root.nsmap[None]} 

to

ns = {'ns' : 'http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15'}

it works. I’m not sure if you can generalize the namespace dictionary to cover both output styles. Maybe xpath’s local-name function (instead of lxml find or findall) is an alternative.

[ChWick]

Okay, just wanted to “clarify” the most obvious mistake. Luckily, I receive the same error using your files. I will examine this and hopefully find the reason!

[ChWick]

Thanks for testing @alexander-winkler . I will close this and merge #259