Fix pyHalo ImportError and update Model-IV simulation logic (#108)

[ApurvaPatil2401]

This PR resolves the ImportError in Model-IV caused by recent API changes in the pyHalo library.

Key changes:

• Migrated from from pyHalo.preset_models import CDM to the factory pattern preset_model_from_name('CDM').
• Updated lens.py to correctly initialize the CDM model using this new pattern.
• Fixed the test block at the bottom of lens.py by adding missing positional arguments (NUMPIX, resolution_coeff) to simple_sim_2.
• Verified the fix by running the script locally in a configured environment.

Fixes #108