EchoDensityProfile: test level-normalised kappa on double-slope RIRs #259

Follow-up from #257. The EDP's kappa ramp (1e2 → 1e5) was only tested on single-slope RIRs. Riccardo notes an amplitude-dependent kappa gave the same results there, but double-slope RIRs were never investigated. Test whether normalising kappa by the signal level helps for double-slope RIRs.

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