Acdh-tei-pyutils

also removes existing tei:back element

fixing 1 and 2
e.g denormalize indices but exclude Schnitzler, Bahr and Wien
`$ denormalize-indices -f "./data/*/*.xml" -i "./data/indices/*.xml" -m ".//*[key]/key" -x ".//tei:title[level='a']/text()" -b pmb2121 -b pmb10815 -b pmb50`