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

Luis, this pull request addresses InSAR issues for 5A-beta. Summary of the request is below. 

Problems observed with ASTER DEM are coming from NEST-core. None of NEST operators will properly handle ASTER at this moment. I am sending you separate email with more details.

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InSAR Stack Overview:

 - lists additional info in table
 - clarifications and update of help,

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.