Refactor download to allow OPERA gslcs #88
Description
For workflows over areas in US where OPERA CSLCs are already exist, and where the user is requesting the same posting (5m x 10m), we can short-circuit the isce3 geocode_slc calls and simply download the CSLC products.
But, we should consider the request to incorporate burst2stack #85 and refactor in a way that opens up more input data sources. Right now isce3 only has S1 and NISAR processors; however, more input sources are expected in the future.