This is a collection of three Kelvin-Helmholtz instability test problems.

Problem 1 comes from Springel 2010, MNRAS, 401, 791:

http://adsabs.harvard.edu/abs/2010MNRAS.401..791S

Problem 2 comes from Robertson et al., 2010, MNRAS, 401, 2463:

http://adsabs.harvard.edu/abs/2010MNRAS.401.2463R

Problem 3 comes from McNally et al., 2012, ApJ, 201, 18:

http://adsabs.harvard.edu/abs/2012ApJS..201...18M

Problem 4 is the same as Problem 3 but halves the perturbation frequency,
so that the transition is smoother.

Problem 5 comes from Lecoanet et al., 2016, MNRAS, 455, 4274:

http://adsabs.harvard.edu/abs/2015arXiv150903630L

These problems are all posed in two dimensions; if you 
compile in 3D, the problem is simply replicated along the last axis.
