better handling of arboratorgrew's specific meta

[kirianguiller]

We should handle better the meta that are arboratorgrew specific.

First, we should have the following :

• AG_user_id : self-explanatory.(Even for parsed sentences, this is the custom name that we can attribute when running a parsing (in the parse settings))
• AG_timestamp : when the sentence was annotated (or parsed) the last time
• AG_parser_id : that would be the exact info of the parser (when was it train, the nickname, the performance, etc). It's only here if the sentence is a parsed one

And then, I think we want that these AG_xxx meta are not exported by default, but an option in the export panel could allow us to do so (useful in case of exporting -> importing usecase, which is often useful when we want to run a script on all sentences of all users).

What do you think @bguil and @khansadaoudi ?

[bguil]

I like the idea. It gives a clear disctinction between AG spectific metadata vs other metadata.
I am wondering how to organize the transition. We should plan an update of the already existing projet.