Implement `H5VL_julea_db_datatype_convert_type_change`

[JBenda]

H5VL_julea_db_datatype_convert_type_change should convert data in buffer A to data with other HDF5 type in buffer B.

Currently, the function does nothing but still returns a pointer to buffer B, which means it's returns a pointer to potential uninitialized memory without notice.

This is just a reminder that this function should be implemented at some point ^^