Fix (for nikon d5100): Read ISO from EXIF differently (similar to inside exiv2's "fixiso" setti...

...ng), and hope this will be the correct way now (works for d5100 at least, please check that other ISO readouts are not broken now!)

This fixes http://darktable.org/redmine/issues/9178 but might be experimental - but it is exactly the same way as exiv2 handles this with the cmd tool!

For D200 this also still works and Sony Nex-C3