For better consistency, now using the grid's size() method instead of getNx()*getNy(). The debug messages in libinterpol2D have been removed after performing the checks (ie that our implementation of Winstral does conserve mass).

the ListonRH algorithm has been tweaked: when only 1 station is available, no detrending is applied (instead of having the detrending returning an error if no default lapse rate has been provided).

Last large restructuring (for the moment): now the spatial interpolations have been put in their own subdirectory. The "RH" interpolation has been renamed as "LISTON_RH" for clarity.