Adjust beam waist treatment to standard convention

The beam waist is defined as the position, where the beam width is minimal. The beam width is yet defined as the place, where the intensity drops to \frac{1}{e^2}, see https://en.wikipedia.org/wiki/Gaussian_beam.

The intensity drops to \frac{1}{e^2}, if x=2\sigma, see the probability distribution function of the normal distribution, e.g. https://en.wikipedia.org/wiki/Normal_distribution. The beam waist therefore defines 2*sigma of the Gaussian beam distribution.

Currently, the beam waist is treated as 1\sigma in the Gaussian function in the DepositionLaser module. This MR modifies this to treat it as two sigma, as by standard convention. The documentation is adjusted accordingly.

Tagging @cscharf