Fixes #393

Add a new setDigitalElevationModel method to deal with non overlapping DEM

See merge request !5

Fixes #375

The computation of index in latitude was erroneous in case of different
resolution between tiles.

The computation of index in latitude was erroneous in case of different
resolution between tiles.

Done:
Algorithm for tiles with change in resolution between North/South edge
of tiles. No change of resolution taken into account between West/East
edge of tiles as known DEM does not have such change.
Algorithm done also for corner zipper tiles.
Test with dummy SRTM
To be done:
Test with real data from SRTM

Algorithm to be defined for DEM tiles without the same resolution.
Done:
Test on anti-meridian added
Test with real data from SRTM (put in comment as needs GDAL)

Add light time correction in atmospheric refraction correction

See merge request !4

Algorithm to be defined for DEM tiles without the same resolution.
Some tests around anti-meridian need to be added.
Some tests with more real data to be added.

Change http to https