GasStripping test fails

The GasStripping test fails from time to time with a too large difference for the surviving number of particles.

The GasStripping test checks how many particles are lost, in the reference these are 1e5 - 99623 = 377. Since this is a stochastic process, the expected error is about sqrt(377) = 20. The allowed error right now in the test is 25. The GasStripping test failed with a difference of 42. To avoid that the test fails randomly from time to time, I propose to set the allowed error to at least 2 sigma=40, perhaps even better 3 sigma=60 (also because the reference of 377 might actually not be a good estimate for the expected number).