H5PTread_packets reads nrecords packets starting at packet number start from a packet table specified by table_id. data is a buffer into which the data should be read.
For a packet table holding variable-length records, the data returned in the buffer will be in form of hvl_t structs, each containing the length of the data and a pointer to it in memory. The memory used by this data must be freed using H5PTfree_vlen_buff.
Returns a non-negative value if successful, otherwise returns a negative value.