Longitudinal profile options for shower particle count in eas_optical

[Areustle]

Add a configuration mechanism to select between multiple particle count generation functions in the eas_optical shower simulation: Greisen & Gaisser-Hillas shower longitudinal profiles.

• Add cherenkov_light_engine options (Greisen, Gaisser-Hillas) to config.py
• Update compute.py to construct EAS object with this config option.
• update cphotang to select particle_count function at construction time
• update cphotang.valid_arrays to compute particle_count
• Unit tests

[Areustle]

This update introduces a new feature to the NuSpaceSim simulator, specifically related to the configuration of the Cherenkov light engine model used in simulations of extensive air showers (EAS). Cherenkov radiation is crucial in detecting high-energy particles, and this update allows for more flexibility and accuracy in modeling the longitudinal profile of particle showers.

Key Changes:

1. Cherenkov Light Engine Model Selection:

• The cherenkov_light_engine field in the Simulation configuration now allows users to select between two different models for simulating the longitudinal profile of the particle showers:
• Greisen: Uses the Greisen profile for modeling the particle count as a function of atmospheric depth. This is now the default option.
• Gaisser-Hillas: Uses the Gaisser-Hillas profile, which can be more accurate for certain types of showers and includes the option to load data from a CONEX table.
• Default: This option is now deprecated, but if specified, it automatically maps to Greisen.

2. Configuration via TOML File:

• Users can now specify which Cherenkov light engine model to use directly in the TOML configuration file.

How to Use the Updated Configuration:

1. Modify the TOML Configuration File:

In your NuSpaceSim configuration TOML file, you can specify the Cherenkov light engine model under the simulation section. Here’s how to set it:

[simulation]
cherenkov_light_engine = "Greisen"  # Default option

Alternatively, you can use the Gaisser-Hillas model:

[simulation]
cherenkov_light_engine = "Gaisser-Hillas"

The literal "Default" will automatically be replaced with "Greisen".

2. Understanding the Models:

• Greisen Model: This is a traditional model for simulating the longitudinal development of particle showers in the atmosphere. It’s generally used for standard Cherenkov radiation simulations.
• Gaisser-Hillas Model: This model is more complex and is used to account for variations in shower profiles, particularly in cases where you want to leverage data from external sources (like CONEX tables). This model might be more accurate for certain conditions, especially for high-energy cosmic ray showers.

3. Implications for Simulation Results:

• Choosing the appropriate model will affect how the shower profile is computed, which can lead to differences in the predicted Cherenkov photon density and angle distribution. This choice is crucial for ensuring that your simulation results are as accurate as possible for the physical scenarios you are studying.