Support sentence-level annotation in external recommender

[reckart]

Is your feature request related to a problem? Please describe. Currently, the external recommender is locked to single-token and multi-token layers. However, I believe there is no reason that we should exclude the recently introduced sentence-level layers.

Describe the solution you'd like

• [x] Allow the external recommender to be chosen for sentence-level layers.
• Actually sent the layer anchoring type to the external recommender such that it might choose a suitable algorithm depending on the annotation task at hand. (open new issue if this is required)

Additional context This was in particular discussed in the context fo the DKPro TC <-> INCEpTION integration.

[reckart]

The PR addressed the first item mentioned in the description. What about the second item?