multilook: support GDAL VRT file

[yunjunz]

Description of proposed changes

• multilook.py:

  • add multilook_gdal() to generate the multilooked lat/lon.rdr files from isce-generated GDAL VRT files (lat/lon.rdr.full.vrt) directly; add example usage in cli.multilook.py.
  • remove the obsolete multilook_matrix() and add its core part as commented code block within multilook_data().
  • multilook_data(): apply cropping of the input data for all methods, as a generic preparation step, to simplify the code laterward.

• utils.readfile.py:

  • read_attribute(): ignore '.mli' for FILE_TYPE, similar to '.full' and '.rdr' in the path
  • get_slice_list(): support dense offsets generated by isce2/topsApp.py, to facilitate the auto reading of read()

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