Use spacy sentence splitter

(SS)

with `NLTKSplitter`:

```
Corpus stats; #docs=100 -- #rels=1345
	#sentences=1056
	edges: #P=351 vs. #N=312
```

with spacy splitter:

```
Corpus stats; #docs=100 -- #rels=1345
	#sentences=1040
	edges: #P=353 vs. #N=351
```

⚠️ So far, it worsens the performance...