improve evaluation:

• recombine combining characters to previous letter char, incorporate that into all metrics and remove metric combining-e-umlauts
• introduce parameter gt_level for historic_latin:
• add multi-character normalizations to historic_latin (historic ligatures and MUFI) when gt_level < 3
• use single-character equivalences beyond NFKC when gt_level==1
• encapsulate counting edit distances into class, use parallel aggregation algorithm for accurate mean and variance estimates
• expose metric, gtlevel and confusion params to standalone CLI eval
• expose confusion size param to OCR-D CLI evaluate