pythia.py vertex smearing values

Using Pythia for full simulation at FCC-ee, specifically Z→ττ at the Z pole, we found a discrepancy between the smearing parameters in the `pythia.py` script ([k4Gen/options/pythia.py](https://github.com/key4hep/k4Gen/blob/643c603589408df920ba437bbb60fff85e54444c/k4Gen/options/pythia.py)) and the cmd card ([FCCee/Generator/Pythia8/p8_ee_Ztautau_ecm91.cmd](https://github.com/HEP-FCC/FCC-config/blob/548c34918312c979ff7a9a80dc399dbb7ab7e666/FCCee/Generator/Pythia8/p8_ee_Ztautau_ecm91.cmd#L31)).

Specifically, in the `pythia.py` script we found:
```python
from Configurables import GaussSmearVertex
smeartool = GaussSmearVertex()
smeartool.xVertexSigma = 0.5*units.mm
smeartool.yVertexSigma = 0.5*units.mm
smeartool.zVertexSigma = 40.0*units.mm
```
while the card has the following parameters:
```
! Vertex smearing :
Beams:allowVertexSpread = on
Beams:sigmaVertexX = 5.96e-3
Beams:sigmaVertexY = 23.8E-6
Beams:sigmaVertexZ = 0.397
```
The card configuration is in the order of the [fcc studies values](https://github.com/HEP-FCC/FCCeePhysicsStudies/blob/e43cc469719c70973b9b8cdbac926b922977ec80/General/README.md#beam-energy-spread).

Looking at the code, it seems that the final smearing is a convolution of both.

Is the pythia.py configuration okay?
Using both, the neutral particle angular resolution is not as good as we expected, and ignoring pythia.py configuration also solves some efficiency degradation.

Compare k4Gen/options/pythia.py with FCCee/Generator/Pythia8/p8_ee_Ztautau_ecm91.cmd, starting at GaussSmearVertex and the card’s vertex-spread settings. Trace where both configurations are applied and check the linked FCC studies values. Done means the project has a confirmed, consistent smearing configuration and the relevant angular-resolution or efficiency behavior is verified.

Written by the indexing model from the issue text.