Add dr return mode to find_dynamic_range()

[Riz1Asif]

This PR adds a mode="dr" option to find_dynamic_range() to return the frequency-dependent dynamic range spectrum directly.

Currently, the function name suggests that dynamic range itself is the main quantity of interest, but the exposed outputs are primarily derived quantities such as amax, amaxd, and boundaries. Returning the DR spectrum directly makes the function behavior more transparent and aligns the API more closely with the underlying implementation.
This PR:

• adds mode="dr"
• updates the docstring

[Copilot]

Pull request overview

Adds an explicit mode="dr" return option to find_dynamic_range() so callers can obtain the frequency-dependent dynamic-range spectrum directly, aligning the public API more closely with the underlying DR-based computation.

Changes:

• Add mode="dr" to return the computed dynamic-range spectrum (as a 2D array stacked with frequencies).
• Update the find_dynamic_range() docstring to describe modes/returns more clearly.
• Normalize frequencies, refractive_index, and (in boundaries mode) absorption_coefficient to NumPy arrays and raise on invalid mode.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.