Support for date and time expression

[kermitt2]

Dates are time measurement, so in the scope of the tool. Currently the recognition of dates and time expressions is limited and there is no normalization for these expressions.

• Dates could already be recognized well by the existing grobid-ner (a standard Named Entity Recognizer based on grobid). Dates can be reliably normalized with the existing GROBID date model. So we can simply integrate/use this existing stuff.
• Time expressions require a specific normalization, in particular to recognize the time zone.
• Expressions combining date and time must be correctly handled and normalized too.