hdf5: figure out how to resolve the Go pointer to Go pointer issue

Created by: kortschak

The GODEBUG=cgocheck=0 incantation is untenable in the long run. So we need to sort out how to deal with types holding pointers.

It is not clear to me how to deal with this using the current approach to calling HDF5 functions where entire values are simply handed off to the library. But anything else involves significant reengineering/reimplementation of the HDF5 code in Go.

Note that the example in the tests that fails this runtime check is already incorrect, since the string field that causes the failure is not correctly round-tripped anyway.