Handle negative BTs in SEVIRI

[ninahakansson]

With the updated calibration for SEVIRI, BT temperatures are some times negative.

The brightness temperatures should have a valid_range.

But also we should handle the negative values. It is the 3.9µm channel that has the most problem. The negative values could be set to the 11µm temperatures or to a minimum value. It could be that values outside vaild_range should be set to _FillValue

[ninahakansson]

For CMSAF this is handled separately by each product. And there are no need to include a fix for this now.