Adiabatic sound wave L1 convergence test

This test consists of a low-amplitude sound wave that is traveling in a periodic
box for one wavelength. To test the rate of convergence, we compare the final
result with the initial condition and calculate an L1 norm from this for
different resolutions.

This test consists of a .xml initial condition specification and a .ini
parameter file, as well as a Python script to calculate the L1 norm.

The script assumes the test has been run 4 times, using 1,000, 8,000, 64,000,
and 512,000 cells, with the resulting snapshots stored in separate folders:
wave1k_r, wave8k_r, wave64k_r, and wave512k_r respectively. To run each
simulation, first generate the initial condition:
SHADOWFAX_RUNDIR/icmaker3d --ncell XXX --setup wave3d.xml --filename ic_wave3d.hdf5
(add the --mode cart flag for a Cartesian initial grid)
and then run the simulation:
SHADOWFAX_RUNDIR/shadowfax3d --params wave3d.ini

The L1_norm.py script will generate a file, L1_wave_r.txt, containing the 1D
equivalent resolution and L1 norm for each resolution.