Shock viscosity

samples/1d-tests/sod_10s (to sod_1000s)

In Makefile.local, set "VISCOSITY=visc_shock" to invoke shock viscosity.
pressure jump 10:1 pressure jump 100:1 pressure jump 1000:1
νsh=4, ν=2 10-3 νsh=3, ν=2 10-3 νsh=3, ν=5 10-4
χ=5 10-4, t=2.4: χ=3 10-4, t=1.9: χ=5 10-5, t=1.5: