Fix model iv py halo import

[Apprentice2907]

Description

Fixes #108

Updates pyHalo imports to use the factory function preset_model_from_name() for compatibility with newer pyHalo versions while maintaining backward compatibility.

Problem

Recent pyHalo versions removed/relocated the PresetModels module, causing:

• ModuleNotFoundError: No module named 'pyHalo.PresetModels.cdm'
• ImportError: cannot import name 'CDM'

Solution

Replace direct imports with version-agnostic factory function:

Before:

from pyHalo.PresetModels.cdm import CDM
from pyHalo.PresetModels.uldm import ULDM

After:

try:
    from pyHalo.preset_models import preset_model_from_name
    CDM = preset_model_from_name("CDM")
    ULDM = preset_model_from_name("ULDM")
except ImportError:
    from pyHalo.PresetModels.cdm import CDM
    from pyHalo.PresetModels.uldm import ULDM

Changes Made

• Updated generate-paired.ipynb
• Updated smooth_image_comparison.ipynb
• No changes to simulation logic or outputs
• Cleared notebook outputs for clean diff

Testing

✅ Notebooks execute without import errors
✅ Works with pyHalo >= 1.x (new API)
✅ Falls back gracefully to pyHalo < 1.x (old API)
✅ No functional changes to simulations

Scope

🔹 Only import statements modified
🔹 No dependency version changes
🔹 No numerical/functional behavior altered