feat(#1579): validate token classification annotations in client

[dcfidalgo]

Closes #1579

Validates the prediction/annotation spans in the client when creating the token classification records. For this, we introduce a new SpanUtils class that also takes care of transforming the spans into tags.

[dcfidalgo]

Comments after the call with @frascuchon :

• from_tags should support BILOU
• add a private attribute to Record classes that hold the SpanUtils instance (this avoids the overhead of computing the char to token mappings every time)
• remove/deprecate token_span and char_id2token_id methods, and private chars2token and tokens2chars attributes
• create a utils module and with it a span_utils.py, move current utils.py to the new utils folder

• [x] support BILOU in from_tags
• [x] add private _span_utils attribute in corresponding token classification record classes
• [x] remove/deprecate old helper methods
• [x] create utils module

still missing:

• [x] add/adapt unit tests

[dcfidalgo]

Ok, this should be ready to review. @frascuchon I left one inline comment, that I think should be corrected before merging.

[frascuchon]

Closing.

Merged from #1709