Correct salvage value calculation logic

[ariane-millot]

Corrects the salvage value formulation.
Previously, the model incorrectly applied:

• The simple linear depreciation formula when discount rate > 0.
• The annuity-based formula when discount rate = 0.

[robertodawid]

Thanks @ariane-millot for pointing this out. Also extending on your proposed change :

In the current version of the model, these conditions are swapped, which would:

• Make the annuity formula apply at DiscountRate = 0 → leads to 0/0 (mathematically invalid).
• Use the linear formula when DiscountRate > 0 → ignores discounting, conceptually wrong.

Test on the CLEWs Demo\res\CO2TAX from the UI

	model (UI)	Eq	OF	NewCapacity	Value
1	SV1,SV2 (=0, >0)	currently	75008.45454	(RE1, PWRSOL, 2026)	0.88
2	SV1,SV2 (>0, =0)	osemosys repo	69679.03367	(RE1, PWRSOL, 2026)	2.17

also tagging: @VedranKapor , @FraGard , @ShravanKumar23 .

As a suggestion, every time the model file is updated, the new version should be tested against a standard OSeMOSYS model/data, for example, UTOPIA, check: https://github.com/OSeMOSYS/OSeMOSYS_GNU_MathProg/blob/master/tests/test_gnu_mathprog.py