Rewrite and explain synopsx.models.tei

[mingarao]

The tei module mechanisms are a little bit confusing at this stage...

• Use more generic functions that can apply to very different tei usages (use for example the Cahier weboai group's best practices on teiHeaders ?)
• At the time we have different types of functions : functions returning map of maps (meta, content), functions returning single maps (content), functions returning nodes : make it clearer how this all works together and maybe use functions naming rules to know what does what
• Make decision about map keys' names and how to use them ('content', 'tei', Dublin Core keys or not, etc.)

[emchateau]

Agree with you about the fact that this part could be clearer.
In most of the case, those functions should built some TEI documents fragments that should be processed after.
I wouldn't decide anything for the keys as they are supposed to be choose by the user.

[mingarao]

in progress

• signatures of the functions linked to restxq url have now the following syntax : queryMyFunction($queryParams) (functions returning meta+content maps)
• signatures of the selecting functions have now the following syntax : getMyItems($queryParams)
• signatures of the item mapping functions have now the following syntax : getMyItemMap($item)