throw error when trying to set_ on read only images

[matthewhanson]

A GeoImage opened by default is readonly, and the set_ functions (e.g., set_gain) will quietly fail. These should throw an error.

[matthewhanson]

For some reason set_nodata still works. Ideally, rather than failing, it would be best if it could set the values for that variable only, even though it doesn't write it to the dataset.