Modify all indices to state SRC and REF as the new Indexparser uses

[obo]

A new branch (soon to merge into SLTev) offers https://github.com/ELITR/SLTev/blob/index-parser/SLTev/index_parser.py, which emits pairs of files given an index. This will resolve ELITR/SLTev#19.

All indices should be ready for this, mentioning SRC and REF file suffixes for all index documents.