test coverage

[keewis]

While having a 100% test coverage is not realistic, it should be above 90%.

In the case of this library, the structure definitions themselves are not really testable (we can run them anyways, but that will only reveal bugs in the helper functions).

[keewis]

as of now we're at 94% test coverage, which is already pretty good. However, there are a few functions / modules I skipped over because writing tests for them seemed tricky:

• ceos_alos2.io.open
• ceos_alos2.sar_image.open_image
• the ceos_alos2.sar_image cli for creating cache files
• the ceos_alos2.sar_trailer module – for this, we need to decide whether being able to read the preview files is actually necessary