Add DEMs with NaNs (or synthesise them) for tests

[erikmannerfelt]

Right now, the Longyearbyen test DEMs are void-free. Therefore, all the tests that pass are only guaranteed to pass with void-free examples. I've had large issues with the coreg functions and I think it is because of nans, so this should be added to further find out what's wrong with them.

I think this would be a good first step toward fixing #61.

[adehecq]

Yes, add NaNs but also masked arrays.

[erikmannerfelt]

Indeed!