WebAnnoTsv3XWriter does not support elevated types

[reckart]

Original issue: webanno/webanno#1539

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Describe the bug
The WebAnnoTsv3XWriter is only capable of exporting span annotations which are direct subtypes of Annotation. I.e. DKPro Core "elevated types" such as POS_NOUN are not supported and do not show up in the export. They do show up in the WebAnno UI though!

To Reproduce

1. Process a document using DKPro Core and produce ah XMI with Sentences, Tokens and POS tags (elevated types).
2. Import that XMI into WebAnno
3. Export it again as WebAnno TSV 3

Please complete the following information:

• Version and build ID: 3.6.3