Main changes are :
- Change fields declaration
- Update metaclass to add field to nmspc instead of _columns
- Add v8's version of compute/reverse/search function of
    disable_per_company field

Add some help

Added 'usable' field in analytic.code. Analytic fields now filter out the codes with usable=False rather than those with children. Added the rel_name and rel_usable options for bound dimensions.

Fields generated by MetaAnalytic now exclude codes with children. Fixed some issues when selecting the parent and dimension of a new analytic code. Added a new 'translate' option.

When creating a code, the parent cannot be selected (readonly) if the code's dimension is not selected (code view) or not a record in the database (dimension view, while creating a new dimension).

Added descriptions to models. Added a 'translate' option to enable/disable translation of analytic code and dimension names (default: False). Fields generated by the metaclasses now have a common placeholder string value. Cleaned up the translation file.

Added a parent-child hierarchy on analytic codes. Fixed a bug where analytic codes without a description couldn't be selected.