Malformed tarballs (data samples)

[neutrinoceros]

Bug summary

Here's the list of sample datasets that currently can't load with yt.load_sample because their respective tarballs do not comply with the de facto rule other data samples are following.

A2052
ExodusII_tests
sedov_tst_0004
Sedov3D
TurbBoxLowRes
GaussianCloud

The rule should be that for a dataset with load key <label> should be compressed in a tarball named <label>.tar.gz which untars to <label>/ (I'll document it explicitly at some point).
Fixing these requires a similar patch to #97.