use rasterio's mask handling approach

[ungarj]

https://github.com/mapbox/rasterio/blob/master/rasterio/_io.pyx#L571

[ungarj]

Modify read() function:

• add masked=False keyword
• default does not return a masked array

Add read_mask() function:

• band behavior (2D/3D arrays) like read()

Maybe add dataset_mask() function to create a 2D mask out of multiple band masks.