Add optional complex field output to coherent and Huygens PSF models

[changlaplace]

Motivation

DeepLens currently computes complex fields internally for both coherent PSF models, but only exposes the final intensity PSF. This prevents users from accessing sensor-plane phase information needed for coherent imaging tasks such as field propagation, interference, phase modulation, and custom wave-optics pipelines.

This PR exposes the already-computed sensor-plane field without introducing a new propagation path.

Changes

• Add return_field=True support to:
  • model="coherent"
  • model="huygens"
• Reuse the existing sensor_field in psf_pupil_prop().
• Return an energy-normalized complex field:
  sum(abs(field) ** 2) ≈ 1.
• Preserve the existing normalized intensity output when return_field=False.
• Document that global phase depends on the optical-path reference, while relative phase remains meaningful.

Compatibility

return_field defaults to False, so existing calls and intensity outputs remain unchanged. No other PSF models or propagation implementations are modified.

Tests

Added focused coverage verifying that:

• both coherent models return complex fields with the expected shape;
• returned fields have unit energy;
• abs(field) ** 2 matches the corresponding default intensity output;
• coherent and Huygens intensity distributions are reasonably consistent under the same optical configuration.

[singer-yang]

Thank you! I will review it as soon as I have time!