PK_ERROR_code_t PK_ATTRIB_ask_nth_double
(
--- received arguments ---
PK_ATTRIB_t     attrib,             --- attribute
int             field_no,           --- field number (>= 0)
int             index,              --- index of double (>= 0)

--- returned arguments ---
double   *const d                   --- nth double.
)


This function returns a single element of the double array from field
field_no of the given attribute.


Specific Errors:
    PK_ERROR_bad_field_number       The attribute does not have a field with
                                    this number
    PK_ERROR_field_of_wrong_type    The field is not a double field
    PK_ERROR_bad_index              The field has no double with this index