Move beam_waist parameter description

For a `converging` beam, track directions would have isotropic distribution (but with a limit on a max angle between the

track and the set beam direction).

The transversal width of the Gaussian beam is defined by the beam waist $w_0$, which describes the minimal beam width. The beam width in turn is defined as the distance between the point of maximum intensity and the point where the intensity drops to $\frac{1}{e^2}$. For a Gaussian-distributed intensity, the intensity drops to $\frac{1}{e^2}$, if $x=2\sigma$, see the probability distribution function of the normal distribution. The beam waist therefore equals $2\sigma$.

**NB**: convention on global time zero for this module contradicts the general convention of the Allpix Squared.

For this module, global t=0 is chosen in such a way that the mean value of temporal distribution is *always* positioned at

*4 standard deviations*  w.r.t. the global t=0.

* `source_position`: a 3D position vector.

* `beam_direction`: a 3D direction vector.

* `beam_geometry`: either `cylindrical` or `converging`

* `beam_waist`: the beam waist $w_0$ is a measure of the width of the transversal beam intensity distribution. It is defined as the minimal beam width. The beam width in turn is defined as the distance between the point of maximum intensity and the point where the intensity drops to $\frac{1}{e^2}$, see [https://en.wikipedia.org/wiki/Gaussian_beam]. For a Gaussian-distributed intensity, 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 equals $2\sigma$. Defaults to 20 um.

* `beam_waist`: parametrises the transversal width of the beam by the beam waist $w_0=2\sigma$ described above. Defaults to 20 um.

* `focal_distance`: needs to be specified for `converging` beam. This distance is *as it would be in air*. In silicon, beam

  shape will effectively stretch along its direction due to refraction and the actual focus will be further away from the

  source.