CorAsvAnnEval: wrap CLI instead of API

[bertsky]

https://github.com/kba/ocrmultieval/blob/5de79f3021b48f83f9cb798a484fd472d21ed94b/ocrmultieval/backends/CorAsvAnnEval.py#L18-L20

I can see it would be hard to repeat everything in ocrd_cor_asv_ann.scripts.compare. So why not instead call the CLI and have it write its report where we want it?

Alternatively, keep the API as it is and emulate Click's context as in:

from ocrd_cor_asv_ann.scripts.compare import cli as standalone_cli

...

def compare_files(self, gt_mediatype, gt_file, ocr_mediatype, ocr_file, pageId):
    outfile = StringIO()
    kwargs = {'output_file': outfile, 
              'gt_file': gt_file,
              'ocr_files': [ocr_file],
              # add more of your non-default choices here, e.g. normalization, gt_level, confusion, histogram
              # if gt_mediatype is plaintext path list, you need to set file_lists=True
    }
    standalone_cli(**kwargs)
    report = outfile.getvalue()
    # dive into this file pair's aggregates
    metrics = report[gt_file + ',' + ocr_file]
    # maybe delete the individual line metrics
    del metrics['lines']
    return self.make_report(gt_file, ocr_file, pageId, **metrics)

[kba]

It didn't even occur to me that I could just use the CLI directly, hence the pass :) Will update.