Checks for EmField :
- checks that the name is uniq in an EmClass
- checks that a relation EmClass <- rel2type -> EmType is uniq

Now on load default fields for EmClass are created. The EmClass.check method create them if they are not present.

Added default fields for EmClass, they are listed by classtype in EmClassType
Fixing some tests (testing that a new EmClass has no fieldgroups)
Fixing the delete_check of EmClass. Now an EmClass can be deleted if it contains only default fieldgroup and default fields

Creating a class RandomEm in EditorialModel/randomem.py

Every backend and MH are now subclasses of dummy ones. This allow to check the type with issubclass.

Make most of the model tests pass (except the tests about the backend calls and set)

Mostly about the changes on types hierarchy

Also do not send change to MH if deletion is not done

=> a type can have multiple superiors (only instance of LeObject can have one)

 use it to re-run ME creation in a new Migration Handler

Code cleaning, commenting etc needed...

Addding a method to indicate the end of a component initialisation phase (used in Model.load and Model.create_component)
Adding a method that dump component attributes (used in Model.create_component and Model.delete_component to call the migration_handler)

No extended checks was done. Only tested if the check passes without syntax error and without failing with no reason

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