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

Associated tests has been added/updated

Checking that internal='object' is reserved for common fields

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

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

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