GH-1449: labeling logic is part of DataPoint

[alanakbik]

This PR refactors the DataPoint class and classes that inherit from it (Token, Sentence, Image, Span, etc.) so that all have the same methods for adding and accessing labels.

• DataPoint base class now defined labeling methods (closes All DataPoint classes should have the same Label logic #1449)
• Labels can no longer be passed to Sentence constructor, so instead of:

sentence_1 = Sentence("this is great", labels=[Label("POSITIVE")])

you should now do:

sentence_1 = Sentence("this is great")
sentence_1.add_label('sentiment', 'POSITIVE')

or:

sentence_1 = Sentence("this is great").add_label('sentiment', 'POSITIVE')

Note that Sentence labels now have a label_type (in the example that's 'sentiment').

• The Corpus method _get_class_to_count is renamed to _count_sentence_labels
• The Corpus method _get_tag_to_count is renamed to _count_token_labels
• Span is now a DataPoint (so it has an embedding and labels)