Feature request: add option to fill missing data

[PrzemoF]

I used Lumix TF5 camera and sometimes the GPS lat/lon data is not present, but the camera uses pressure sensor to determine altitude. gpscorrelate treats those photos as "GPS Data present" even if there is no lat/lon. I'm not sure what's the best solution: check for what's missing and fill it from the GPS file or allow to mark photos with incomplete data as without any data to allow easy correlation. Currently I'm stripping the data first and then do normal correlation.