Fix crash when no dem url provided.
Merge pull request #212 from khogenso/devel

Fix crash when no dem url provided.
Do not include url if we don't have it
Merge pull request #213 from khogenso/devel

Do not include DEM/tropo url if we don't have it
Merge pull request #37 from asfadmin/devel

Pull latest changes
Merge pull request #38 from rudigens/master

Pull latest changes