Writer from the U-layer to UMR

[michnov]

In the conversion pipeline of Czech documents from tectogrammatical representation to UMR, we propose an intermediate U-layer, which should be in terms of the structure and inventory of relations and attributes as close to the final UMR as possible, yet still represented in the PML (XML-based) format displayable and editable in TrEd.

The first part of the conversion pipeline should be provided by Treex scenarios.
For the full conversion pipeline, we still need the second part: converter from the U-layer to the UMR format.
The resulting UMR format should be valid with respect to the UMR validator in this repository as well as ready to be visualized by UMRGraphViz.