parallel options

[falkamelung]

Hi @forrestfwilliams
That works very good. Thank you for providing this! Here some suggestions:

• it would be good to have two --parallel options. If you have all bursts it seems to do serial SAFE file creation, one after the other, only using one CPU. Using multiple CPU for multiple parallel jobs could speed things up significantly.
• An option for downloading only would also be good. On many HPC systems you can use the head node for downloading but not for processing which the merging/concatenation is. I am actually downloading using asf_search and the use burst2stack for SAFE file creation.
• A second parallel option for downloading would also be good. Get first all bursts and once you have all of them create the SAFE files (as I am doing it).