Factorize setting of source anchor and parents

[jecisc]

Currently when we create an entity we fill by hand the parent and source anchor but we should be able to simplify this by checking the type of the top context and the meta description of the entity to create to simplify the parser

[NicolasAnquetil]

That would require a unified way to implement parsers, all with the same way of handling context.

We could start to implement traits to ease development of parsers...
(see also #18)

[jecisc]

For now I was just thinking about implementing one simple method to do that. That could go in Famix to help with future parser indeed