Compressed HDUs: NaN values are replaced with extremely negative numbers

[ddobie]

As part of compression, NaN values are replaced with extremely negative numbers.

I'm not sure if this can be handled in a sensible way on the compression side of things - e.g. setting them to something very positive/negative isn't sensible, setting them to zero could be misleading.

[ddobie]

This is essentially close because we crop tightly enough that here are no NaNs. However the issue does technically persist, so I'm leaving it open.