Inline operator support

Currently Yirgacheffe overloads `__add__` and `__radd__` but not `__iadd__` (and similarly for sub, mul, etc.). It might be nice to be able to write:

```python3
with yg.read_raster("blah.tif") as layer:
   layer *= 2
   layer.to_geotiff("doubled.tif")
```
