-/** \brief reads block from metafile
- \param mf metafile handle
- \param no block position
- \param offset offset within block
- \param nbytes no of bytes to read (0 for whole block)
- \param buf content (filled with data if OK)
- \retval 0 block partially read
- \retval 1 block fully read
- \retval -1 block could not be read due to error
- */