Add documentation about GSL random number generators
We use the GSL random number generators in a few places. By default the seed is always the same, but this can actually be changed using an environment variable. See:
https://www.gnu.org/software/gsl/doc/html/rng.html#c.GSL_RNG_SEED
This information should be added to the documentation and should be mentioned in a few input options, like RandomVelocityTemp
.