Fix multiple file loading for metop l1b data.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Orbit doesn't have to be a string anymore.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>
Add 10 km to the area extent on each side, to avoid tangent cases.

Signed-off-by: Martin Raspaud <martin.raspaud@smhi.se>