subordinates implementation is note tested

Not tested because of bugs in EmField

Changed comments about EmFieldGroup.fields()

It takes an EmComponent child class as parameter and return an sqlalchemy Table

Passe tous les tests

Récupération dans EmComponent directement

Suppression du init et passage des classes plutôt que les instances pour les fieldtypes dans le _fields

Now _fields and table MUST be class attribute.
Its important to note that _fields now consist in a list of (name, ftype) tuples and that ftype is a EmFieldType class and NOT an EmFieldType instance

Cleaning code for EmComponent
Optimizing tests for EmComponent and EmFieldGroup (running initDb in setUpModule and make a backup copy, the backup is restored in setUp)

- Now except for the modify_rank tests test_component didn't fails
- Added simple integer operation capabilities to EmFieldType_integer ( int cast, +=, -=, *=, /=, +, -, *, /, %, ...)