Move CemaNeige daily gradients outside DataAltiExtrapolation_Valery

Move GradT_Valery2010() in an utils file, outside DataAltiExtrapolation_Valery() in order to make it more readable.

In additon, set GradP_Valery2010 as a vector and GradT_Valery2010 as a data.frame instead of functions.

Rename GradT_Valery2010 into .GradT_Valery2010.