Add function to_rasterio + fix #31

As discussed in #31 (closed), I propose to add a function to_rasterio which will return image as numpy array and a dict containing metadata required to later write the image using rasterio.
Here is an example of a rasterio metadata dict (A.K.A. rasterio object's profile) :

{
 'blockxsize': 128,
 'blockysize': 128,
 'compress': 'lzw',
 'count': 1,
 'crs': CRS.from_epsg(4326),
 'driver': 'GTiff',
 'dtype': 'int32',
 'width': 43200,
 'height': 17400,
 'interleave': 'band',
 'nodata': -9999.0,
 'tiled': True,
 'transform':  
    Affine(0.008333333333333333, 0.0, -180.0,
           0.0, -0.008333333333333333, 85.0)
}