move the only torchgeo utils we use into separate module to be independant from torchgeo during install. See #23

change default value for quantization from true to false as it can lead to some crashes (to heavy on ram ?)

get mean and sd by band with random subset if no metadata to avoid loading the entire raster (default hardcoded behaviour for now)

from rasterio 1.4, merge() method does not take `datasets` parameter but `sources` parameter