Support multary relation

[wajdikhattel]

The apply function already supports a list of mentions, so why not give the possibility to get a relation of multiple mentions.

This is needed in order for fonduer to support multary candidates.

[senwu]

LGTM.

Two comments:

1. Can you give few examples about the features generated by this PR?
2. Did you test this PR?

[wajdikhattel]

So I only tested this directly with fonduer, and I get around 50 new textual features when running the
test_multary_relation_feature_extraction at test_features.py

It's hard to get a screenshot of the example, but basically this will allow us to treat a relation between multiple mentions instead of just two.

What I can suggest is to run that test while un-commenting the textual features part in the test_features and in the featues/feature_libs/textual_features.py
And basically you'll notice that we have more features

The first n_textual_features is the one from the test_binary_relation_feature_extraction and the second is from the test_multary_relation_feature_extraction

[senwu]

Awesome! We can think about merging the apply_multary_relation and apply_relation later. This change might not backward compatible.