add some plotting to 3d examples

[rjleveque]

All of the examples for 3d have a setplot.py that does nothing but print a warning that plotting isn't implemented in 3d. Eventually it would be nice to have 3d plotting, but in the meantime we could at least add some plotting of gauges and of 1d or 2d slices of the 3d data, which I could be done fairly easily within setplot.py e.g. by specifying a plot_var function that returns a 1d or 2d array.

For euler_3d_radial we could also add a 1drad subdirectory that computes a reference solution and then make a scatter plot of the 3d results compared to this (as in acoustics_2d_radial for example) .