detect if running examples from pytest in the recommended way

[jshipton]

See here:
https://doc.pytest.org/en/latest/example/simple.html#detect-if-running-from-within-a-pytest-run

[JDBetteridge]

Is the idea that this would replace the

if '--running-tests' in sys.argv:
    nlayers = 5
    columns = 30
    tmax = dt
    dumpfreq = 1
else:
    nlayers = 10
    columns = 150
    tmax = 3600.
    dumpfreq = int(tmax / (2*dt))

pattern in the examples?

[jshipton]

Resolved by #541