v.1.0.11 DJI support relative height XMP tag

Added the option to use relative height for DJI and EXIF importers

The relative Height is a tag which stores the relative height to the starting point of the drone. This is done with barometer and other sensors.
As there seem to be sometimes problems with the GPS height, I added the possibility to use the relative height.
For missions on sea or near the coastline (where a plane mapper of height 0.0 is used) you simply can state the approxmite height above the sea level you started your drone.