Use only simple string to define attributes

In contrast to properties, attributes are defined in the schema, so their names and numbers are set. Therefore, it makes very little sense to be able to specify attributes with patterns or enumerations as is done in some examples:
* https://github.com/buildingSMART/IDS/blob/master/Documentation/testcases/attribute/pass-name_restrictions_will_match_any_result_3_3.ids
* https://github.com/buildingSMART/IDS/blob/master/Documentation/testcases/attribute/pass-an_optional_facet_always_passes_regardless_of_outcome_2_2.ids
* https://github.com/buildingSMART/IDS/blob/master/Documentation/testcases/attribute/pass-name_restrictions_will_match_any_result_3_3.ids

Also the cardinality for attributes is meaningless.