hdf5: first stab at hdf5-1.10 support

[sbinet]

This CL is a first step towards supporting new features and APIs
of HDF5 v1.10.

Right now, anecdotal evidence shows that HDF5 v1.8 is still massively
deployed so support for 1.10 has to be opt-in, by way of an optional
build tag aptly named "hdf5_v1.10"

Updates #16.

Please take a look.

[donkahlero]

That looks good. And to confirm it even further. We are running this lib for quite some time now on HDF5 v1.10.1 without any issues.

[sbinet]

I am sure 1.10.x is great :)
no argument about that.

the issue is twofold:

• IIUC, the way travis works, we can't reliably test gonum/hdf5 against both hdf5-1.8.x and hdf5-1.10.x: we have to pick one.
• given the limited dev resources in Gonum and in gonum/hdf5 in particular, I don't think it's doable to correctly support both.

I'd be inclined to say we support hdf5-1.8.x (or try to) and accept PRs for 1.10.x.
when/if the HDFGroup declares 1.8.x deprecated, we say we drop it as well and enable 1.10.x by default.
alternatively, we could ask our users if they'd be willing to drop explicit support for 1.8.x and focus on 1.10.x (but this would involve some .travis.yml massaging to get the correct version installed there, I suppose...)

this PR was just for me to see how one could achieve support for 1.10.x w/o disturbing 1.8.x and lay some ground infra work down :)

[kortschak]

IIUC, the way travis works, we can't reliably test gonum/hdf5 against both hdf5-1.8.x and hdf5-1.10.x: we have to pick one.

I don't think this is true. We could use an environment variable to choose which is installed and built against.

[kortschak]

Is it worth adding this to the pre-1.10 being an error-returning noop?

[kortschak]

I think the convention more broadly has become // +build ....

[gustafj]

IIUC, the way travis works, we can't reliably test gonum/hdf5 against both hdf5-1.8.x and hdf5-1.10.x: we have to pick one.

I don't think this is true. We could use an environment variable to choose which is installed and built against.

You can use the matrix expansion support in Travis to build against different versions.
I cant say how it would be done in the hdf5 case though (don't know enough about the your build setup).

Some references:
https://docs.travis-ci.com/user/build-stages/matrix-expansion/
https://docs.travis-ci.com/user/customizing-the-build/#Build-Matrix