Operations requiring only Raster.data.mask load the entire data array

[rhugonnet]

We can use rasterio.read_masks() to only get the mask and perform operations requiring only the data mask (and not the data array) in a much more memory-efficient way.
This includes for example:

• subsample() and any functionality that uses it like to_points(),
• crop() with option to crop only to nodata (not yet implemented, see Add a functionality to crop raster to valid extent (remove nodata cols/rows on sides) #400).

Starting a _load_only_mask function in #501...