Use the new index syntax

[obo]

SLTIndexParser now understands file suffixes specified in index files (https://github.com/ELITR/SLTev#parsing-index-files).

What needs to be done is:

• Modify SLTev to use the indexparser as its own internal library to actually parse indices
• Move the indices we are going to report in D1.6 to this new format
• Document the new index syntax in indices/README.md, not just in SLTev repo