InSAR update for 'insar stack overview' and 'external dem' issues

InSAR Stack Overview:

 - lists additional info in table

Support of ASTER, External and other DEMs in TOPO phase removal:

 - now 'partially' supports External DEM, limitations listed in jLinda repo,
   and in operators help;

 - reported issues with ASTER DEM are related to problems NEST-core and not jLinda.
Merge pull request #13 from pmar/insar_issues

InSAR update for 'insar stack overview' and 'external dem' issues