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

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

Added a dummy EmFieldType_icon
Added a test for EmType deletion with superiors

Changed beahvior : when a 'non-change' is asked (like +0 -0 or set ranke to actual rank) don't raise an Error

Implemented __hash__ and __eq__ for EmComponent (based on class name and uid) to test equality betwenn two instance. Allows to use "instance in instance_list" syntax

Behaviour changed for EmComponent and childs create method :
	- Takes care of duplicated names at creation
	- Handle all the checks in EmComponent method
Factorization of EmField.get_class_table() method
Added a @property method class_table_name to EmClass

Added docstrings comments to tests (for fancy output with -v ).
Fixed strange hidden failure that make tests stops on non failed tests with -f option. Those false failure seems to comes from with self.subTest blocs with functions calls that does assertion.

Moved the database initialisation functionnality to sqlsetup and fixed all the code to handle the sqlwrapper deletion

Added hierarchy tests

The EmField version cannot be tested yet (drop column not yet implemented for sqlite in Database.sqlalter)

Also add a new function is sqlutils to execute ddl

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