nominalFocalLength is undefined for zoom lenses

[jamesmosys]

Zoom lenses are defined by a nominal minimum and maximum focal length. Currently the static packet only support a single number for this - which covers the prime lens case e.g. 50mm.

[JGoldstone]

What I could do is to set the nominal focal length for a zoom lens if the value during the sequence never changed more than 0.5mm; if it stayed within that range, basically just registering a zoom that wasn't being used as a zoom, I'd take the average value. But that's pretty complex semantics. For the first cut at PR I just won't set the attribute if the values differ at all.