Duplicate HXL when exporting GPS questions #209

Really a https://github.com/kobotoolbox/formpack/ issue, but kept here for continuity with the source issue: kobotoolbox/kpi#1479

For the location questions, if UNHCR is adamant, then I would offer to hard-code +lat, +lon, etc. in our exports when a HXL tag is present on the GPS question. Example export header, given a GPS question called label with the HXL tag #tag would look like:

label | _label_latitude | _label_longitude |  _label_altitude | _label_precision
#tag  | #tag+lat        | #tag+lon         |  #tag+elevation  | #tag+accuracy   

This would be simple concatenation, so if the user tagged the GPS question with #tag+attr, then the split columns would show #tag+attr+lat, #tag+attr+lon, ...

Don't forget: confirm that the HXL standard actually matches the location attributes provided in the UNHCR feedback (updated elevation per kobotoolbox/kpi#1479 (comment))