Add support for NAF output in alpino

[vanatteveldt]

It would be good to be able to output alpino in NAF format from nlpipe.

We can hopefully use the code from https://github.com/cltl/morphosyntactic_parser_nl to create the file. Complication is that it uses both the xml and dependency output.

I guess the xml->dependency step should be really cheap, so we could call alpino in the convert routine, which means that the server should have access to alpino as well.

Alternatively, we can do it as an actual pipeline and have a xml->naf step after alpino (but there's no support for actual pipelining yet :))

(and related, create a simple server for the alpino docker that can do the xml->dependencies step and/or accepts end_hook arguments)

[vanatteveldt]

(cc @antske )