H5Pset_libver_bounds compatibility option

HDF support,

Can you please provide a new file compatibility option in
the H5Pset_libver_bounds function? Currently the only available options
for the "low" argument are "earliest" and "latest". It would be good to
have a middle alternative requesting the superblock version and internal
object versions to match the current 1.8.19 library, or more generally to
match the 1.8 series around the time of introduction of HDF5 1.10.

I have several reasons for asking. One is to try to get a good format
match with large numbers of netcdf-4 files that my agency previously
published using the HDF5 1.8 series. Also I think that migration from 1.8
to 1.10 will be more generally acceptable if 1.10 includes a 1.8 format
matching option.

Also, I am suspicious that files created with low = "earliest" may have
reduced read back performance in some cases. I do not have current
examples or enough information about effects of the different internal
object versions to be able to assess this properly. However, performance
issues were hinted in the recent discussion about many attributes and many

Thanks for your consideration.