specify LAMBX LAMBY outputs in get_sim2_daily

In get_sim2_daily: LAMBX and LAMBY outputs are defined as follow in the docstring:

#' - **`LAMBX`**: Easting in Lambert II étendu (hm)
#' - **`LAMBY`**: Northing in Lambert II étendu (hm)

We could more explicitly explain that they provide the centroids of each pixel.