PK_ERROR_code_t     PK_ATTRIB_ask_named_vectors
(
--- received arguments ---
PK_ATTRIB_t         attrib,         --- attribute
const char         *field_name,     --- field name

--- returned arguments ---
int          *const n_vectors,      --- number of vectors (>= 0)
PK_VECTOR_t **const vectors         --- vectors (optional)
)


This function returns the array of vectors from field field_name
of the given attribute.


Specific Errors:
    PK_ERROR_bad_name               The attribute does not have a field with
                                    this name
    PK_ERROR_field_of_wrong_type    The field is not a vector, coordinate
                                    or direction field


The field should be of type vector, coordinate or direction.

The output array vectors is optional.  If it is set to NULL, the function
will only return the number of vectors in the field.