Set initial parameters and fix tt in snowmelt model

[raoulcollenteur]

Describe the proposed feature

A new study suggest some good (initial) values for the snowmelt module:

https://egusphere.copernicus.org/preprints/2025/egusphere-2025-1214/egusphere-2025-1214.pdf

Also, in my experience estimating parameter tt is diffucult and fixing it to 0 degrees celcius is often better and makes sense (also supported by this study).

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