unloadable or broken datasets

[neutrinoceros]

We have identified at least 3 datasets from https://yt-project.org/data/ that seem impossible to load (either using yt.load or yt.load_sample):

• ramses_extra_fields (targeted for removal in Remove unused RAMSES dataset #102, but could in fact be fixed by repackaging the tarball according to @cphyc)
• output_00080_halos : I couldn't find a working way to load this one (apparently this is a very rare case of a two-stages load ?)
• CompactObjects: this one isn't used anywhere and seems to be unloadable currently (fails either with YTUnidentifiedDataFormat if passed the base directory or a segfault if passed any of the embedded files). I wonder wether it can be salvaged.

[cphyc]

For output_00080_halos, you first need to load output_00080 which you then feed as follows

ds = yt.load("output_00080")
halos = yt.load(output_00080_halos, parent_ds=ds)

I don't see a simple way to support it via the load_sample mechanism.

[neutrinoceros]

Indeed. I think it warrants a creating a blacklist. The better solution would be to make it loadable in a single step (with kwargs ?), though right now I'm not 100% convinced it's feasible.