Add precision in PE_Oudin doc

The Oudin paper used meteorological data at the station level to calculate PE. This is not specified but it is likely that point PE data were then basin averaged. (this is also what we do in our team when we use SAFRAN gridded data)

We should therefore add an advice to do that in the Details of the PE_Oudin doc. I propose the following:

To calculate basin-wide Oudin potential evapotranspiration, it is advised, when possible, to use either station temperature or gridded temperature data to calculate PE and then average these PE values at the basin scale.