At creation if no rank specified the component will be place last, else you can pass an integer representing an absolute rank or two specials strings 'last' or 'first'

EmComponent::check() always return True but rearrange ranks if they are not continuous

comment selected_fields because it clashes with ._fields[]
 => we have to rethink of ._fields[] and fieldtypes

Now Model as an attribute named migration_handler
DummyMigrationHandler has only one method : register_change

Model._components now contains only one dict ( 'uids' key ) and the others are now list of EmComponent instances
Model now handle EVERY access to the _components dict

There is now two methods Model.name_from_emclass and emclass_from_name that allows to access the Model.components array