Add conversational entity linking into REL

[stefsmeets]

This PR implements converstational entity linking from @hideaki-j into REL.

It's very much a work in progress. So far I got the server working, see ./scripts/test_server.py for an example.

Todo

• Align conv EL with REL itself (re-use parts where possible)
• ConvEL has its own mention detection using Bert, this should be aligned with @eriktks work.
• Figure out what the correct API for interacting with the server should be.
• Update documentation (setup, paths etc, how much should this be part of REL?)

[stefsmeets]

Implementation using #147: https://github.com/informagi/REL/tree/fastapi-conv-el

[stefsmeets]

Dropping this in favour of #150