Remove `compare` worker.

The compare worker was only used internally. It has been removed from the
official tree.
Rework stopword filtering.

The 'stopwords' parameter now specifies a list of stopwords. The pre-defined
lists are now available with the parameter 'stopwords_preset'.
Add support for textspan windows in cooccurrence networks.
Export token count for nodes into graphml.
Use word forms instead of label for stopword filtering.
Reflect renaming from named_entity to entity in tcflib.
Revert change that introduced slower ngrams.
Remove 'type' option for tcf.Graph.
Adapt to change in TCFlib API.
Remove unfinished loglikelihood edge weight method.
Handle newly introduced LoopError.

The tcf.Graph API now raises a LoopError when trying to add loops to the
graph. These cases are now handled.
Add special option 'sentence' for window boundaries.
Add 'unique' option to cooccurrence worker.

Using this option, an edge for two tokens is only added if it has not been
added for the same two tokens before.