PK_ERROR_code_t   PK_GROUP_merge_entities
(
--- received arguments ---
PK_GROUP_t        group,            --- group
int               n_entities,       --- number of entities (>=0)
const PK_ENTITY_t entities[],       --- entities

--- returned arguments ---
int        *const n_added           --- number of entities added to group (>=0)
)


This function merges the given array of entities into the given group.


Specific Errors:
    PK_ERROR_not_in_same_part       (SERIOUS) some entity is not in the same
                                    part as the group
    PK_ERROR_wrong_class_for_group  (SERIOUS) some entity is of a class not
                                    allowed in the group


The array entities may contain duplicates, and any of its elements may
already be in the group. The updated group will not contain duplicates.