[WIP] Documentation of the AST

[Halbaroth]

This PR aims to document as much as possible the current AST of Alt-Ergo. Some cleaning works has been done while this marvelous travel :)

• Remove an id used by the graphical interface. This id was set to 0 everywhere and the AST of library shouldn't contain such information.

[CLAassistant]

All committers have signed the CLA.