Calculate irradiance

[jtgasparik]

1. Function added to calculate the normalized incident irradiance from previously computed parameters $\tau$ and $\sigma$.
2. Function to plot the normalized incident irradiance and scale according to peak irradiance.

NOTE: the scaling will have to be modified for evaporative particles. currently, the scaling is purely for visualization purposes.

[Copilot]

Pull request overview

This PR adds support for computing and visualizing the (Gaussian-model) normalized incident irradiance derived from previously estimated Moteki & Kondo parameters (τ and σ), and extends the existing incident-irradiance plot to include an irradiance overlay.

Changes:

• Added compute_normalized_incident_irradiance_moteki_kondo() to compute (I(t)/I_0) from tau_best and sigma_hat.
• Updated plot_incident_irradiance() to plot a (scaled) incident-irradiance curve alongside the scattering signal.
• Extended NDM tests to exercise the new irradiance computation and made small test tweaks in visualization tests.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 5 comments.

File	Description
pysp2/util/normalized_derivative_method.py	Adds the normalized irradiance computation helper and overlays the irradiance curve in the incident-irradiance plot.
tests/test_ndm.py	Adds test coverage that exercises the new normalized irradiance helper against an example waveform.
tests/test_vis.py	Minor tweaks to the visualization test setup (event selection comment/whitespace).

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[rcjackson]

See my comments responding to copilot.

[rcjackson]

I have two minor changes to request. Good job on this!