Description of proposed changes

• multilook: add --search-win --xcorr-win --margin to feed options from pycuampcor, for exact multilooking file size

• multilook: add --off-file for both CPU and GPU ampcor, to use existing offset file as the size reference

• readfile.read_isce_xml: support isce configuration file, e.g. topsApp.xml

Reminders

• Pass Pre-commit check (green)
• Pass Codacy code review (green)
• Pass Circle CI test (green)
• Make sure that your code follows our style. Use the other functions/files as a basis.
• If modifying functionality, describe changes to function behavior and arguments in a comment below the function declaration.
• If adding new functionality, add a detailed description to the documentation and/or an example.