T gradient in DataAltiExtrapolation_Valery only valid in Northern hemisphere

I suspect that the DataAltiExtrapolation_Valery T gradient defined in GradT_Valery2010() is only valid for the Northern hemisphere.

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To avoid this issue, I propose the following:

• we remove the GradT_Valery2010 object from the DataAltiExtrapolation_Valery code

• we store our GradT_Valery2010 object in the package such as Param_Sets_GR4J

• in the doc of GradT_Valery2010, we specify for which area it was obtained (Northern hemisphere, and I guess France (to verify))

• we add an argument called GradT, by default set to GradT_Valery2010, to the DataAltiExtrapolation_Valery function

• we also add an argument call GradP in function DataAltiExtrapolation_Valery, set by default to 0.00041 ### value from Valery PhD thesis page 126

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