Exiv2

This originally started as a bug report that my Tamron 150-600mm f/5-6.3 is not detected by exiv2 at all, because the lens reports a maximum aperture of 5.6 instead of 6.3, presumably so that the camera's AF still works. But quickly I discovered that for most lens IDs with more than one lens the maximum aperture is not necessary (or useful) to distinguish the lenses. Only for lens IDs 10 and 26 there are lenses with the same focal length but different apertures. Lens IDs 137, 161 and 173 also have lenses with the same focal length, but for those the aperture is identical too, so they cannot be distinguished by the current implementation.

Therefore, the attached patch changes the implementation so that only lens IDs 10 and 26 use focal length and aperture to try to determine the correct lens description, whereas all others use only the focal length. With this patch my Tamron is detected correctly, and this should also fix #926, #927 and #938.

#947 is not fixed by this patch, as in this case both lenses have the same focal length and maximum aperture. Maybe these lenses can be distinguished by other lens-dependent tags (perhaps minimum aperture?), but this would require images taken with both lenses (and ideally the same camera) for further investigation.