[CWEP002 migration] support drop_relation_type/drop_relation_definition/drop_attribute for computed attributes

[CWEP002 migration] support add_relation_type/add_attribute for computed attributes Related to #3546717.

[CWEP002] properly handle serialization of computed attributes Until this cset, only serialization of computed attribute's formula was handled (thanks to the relation definition's properties dict). We now: * properly serialize/deserialize attribute's formula * test first if the database has been migrated and contains related column or not Related to #3546717

[CWEP002] Add support for computed attribute synchronization Related to #3546717. Test and handle the behaviour with several formulas and identified use cases. To do so add a birth year and a computed attribute age to Person in the test schema.

[CWEP002] Add schema finalization checks for computed attributes ``finalize_computed_attributes`` essentially checks that computed attributes types match with the type declaration of the attribute. Related to #3546717.

[CWEP002] Account for attribute formula in schema bootstrap This isn't enough to have computed attribute support, it is only in order not to crash when yams 0.40 is used. Related to #3546717.

[CWEP002 migration] support sync_schema_props_perms for computed relations

[CWEP002 migration] support add_relation_type for computed relations Related to #3546717.

[CWEP002 migration] support drop_relation_type for computed relations Related to #3546717.

[CWEP002 migration] properly raise exception on (add|drop)_relation_definition for computed relation Related to #3546717

[hooks] do not abuse of inheritance for CWRType hooks Just copy the selector to ease readability.

[schema serial] add some comments

[schema] define full_rql on RQLExpression class without it, we may have an error while displaying error which occured during some expression initialization because this attribute is not yet defined while used in the __str__ method.

[schema] properly raise BadSchemaDefinition when some expression mains variables may not be guessed

[CWEP002] properly handle serialization of computed relations We now: * have CWComputedRelation in the bootstrap schema to store computed relations * properly serialize/deserialize it * test first if the database has been migrated and contains the related table Related to #3546717 [jcr: adjust unittest_querier to pass with the added entity type]