multilook: geometry support for CPU/GPU ampcor products

[yunjunz]

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

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