[Index] simplify index definitions and use index fields

[dariok]

Instead of indexing e.g. tei:note, tei:head etc. separately, using the local-name() of the containing element as a computed field might be easier.
This is more flexible as additional element names do not have to be added to the index definition.

It is interesting to check the size of the index and the time required for indexing for improvements or possible new bottlenecks.