[Proj] conversion latlong to UTM (& vice versa)

[Greg Troxel]

  I believe it is not possible to derive from the UTM coordinates the
  zone number.

I concur, and I'd go further:

It's reasonable to say "x E y N (UTM zone 19)", and "x E y N zone 19
(UTM)", but "x E y N (UTM)" makes about as much sense as saying that
the coordinates of a point are "42 north WGS84".  So really if you
convert to UTM and drop the zone, you are losing information and you
have to infer to go back.

That said, my typical values take me to low eastings in zone 19 nd
high in zone 18, so given a history, a start zone, a measurement rate,
and a speed limit one very likely can infer the zone.  But really it
would be better to pass a 3-tuple rather than pair.