Use rasterio's read/write masks

[adehecq]

Is your feature request related to a problem? Please describe.
Currently, masking is only handled by nodata. This can be a problem for example for uint8 arrays, for which it might not be possible to "waste" a value.

Describe the solution you'd like
We could use an additional channel for masking, using rasterio's functionality: https://rasterio.readthedocs.io/en/latest/topics/masks.html
This would require calling read_mask and save_mask whenever needed.

Describe alternatives you've considered
None

[atedstone]

Masking now implemented(#235, #271)