* drop .rel / .sql file support
* deprecate MetaEntityType, MetaUserEntityType, MetaRelationType, MetaUserRelationType, MetaAttributeRelationType
* deprecate import_erschema (use python import instead) / defined_types (use context.defined instead)
* deprecate implicit context defined when loading schema module: should move towards proper (importable) python module
* simplify loading api consequently (though a lot of sutff remains to handle deprecation / bw compat properly)