Put in TVD RK3 advance for Euler solver
Clean up RK3 advance function
Have 2nd order in space working, but with kludge at refinement jumps

The kludge is the same as was used in the first version of rabbit. When
the stencil straddles a refinement boundary, there are formally two
mistakes being made:

1. Piecewise constant prolongation is used on the coarse side of the
   refinement jump; it should be interpolated down to finer cells

2. The last coarse zone is arbitrarily copied to the missing 'shell'
   zone containing the two finer ones on the finer side of the jump;
   data should be averaged from the finer ones into the shell zone

|  |  |
|  |  |  | | | |
         ^
^        | refinement jump
|
coarse side