Spaces between `B-` entities should be labelled as `O`

Ref: https://redmine.teklia.com/issues/5503#note-3

There is an issue with the Document.char_labels method which impacts the atr-ner-eval nerval evaluation command.

There is an error with character-level labels when two B- tokens of the same entity follow each other. Since the B- tokens are separate entities, the space between them should be labelled O.

• Current output

>>> doc = Document("dog B-Animal\ncat B-Animal")
>>> doc.chars
['d', 'o', 'g', ' ', 'c', 'a', 't']
>>> doc.char_labels
['B-Animal', 'I-Animal', 'I-Animal', 'B-Animal', 'B-Animal', 'I-Animal', 'I-Animal']

• Expected output

>>> doc = Document("dog B-Animal\ncat B-Animal")
>>> doc.chars
['d', 'o', 'g', ' ', 'c', 'a', 't']
>>> doc.char_labels
['B-Animal', 'I-Animal', 'I-Animal', 'O', 'B-Animal', 'I-Animal', 'I-Animal']