New DEM handler without OSSIM.

Sub issue of #1506 (closed) remove OSSIM.

The current otb::DEMHandler class is an adaptater class for OSSIM DEMs. The objective is to refactor this class to use GDAL instead.

As @jmichel explains in comments, the RPC transform class in GDAL accepts a path to a DEM file, which is then opened and used internally by the RPC class. The first idea was to encapsulate this class in OTB. But after investigations, it appears that the DEM management functions are not part of GDAL's public API. Therefore, it is not possible to access the DEM interpolation as initially planned.

The new approach is a based on the RasterIO from GDAL. A 2x2 window is extracted around the point of interest and the height is interpolated, above ellipsoid or above mean sea level.

More information on the new implementation of the DEMHandler is available on the wiki