HSE BC issue with compressible_rk?

[zingale]

The RT problem seems to get pushed up from the boundary when run with compressible_rk. Other hydro problems are fine, so this is a gravity or HSE BC problem.

[zingale]

some findings:

• running 2nd order RK seems to work okay (but we need to use a smaller CFL because of instability):

./pyro.py compressible_rk rt inputs.rt compressible.temporal_order=2 vis.dovis=0 driver.cfl=0.4

• using reflect BCs top and bottom seem to keep it centered vertically, but there is some distortion

./pyro.py compressible_rk rt inputs.rt mesh.ylboundary=reflect mesh.yrboundary=reflect vis.dovis=0

[zingale]

This is a CFL issue. Running with CFL < 1/d (d = dimensionality) fixes it