diff: support unw/off - timeseries

[yunjunz]

Description of proposed changes

• diff.py:

  • support the differencing between a *.unw (or offset file with FILE_TYPE of displacement) file and a time-series file, as used frequently in phase correction procedures, by adding a sub-function diff_ifgram_and_timeseries()
  • split diff_timeseries() and diff_ifgram_stack() out of diff_file() for clarity.
  • prepare isce metadata file, to be consistent with mask.py and subset.py

• call diff.py to handle the correction procedure in the following scripts:

  • iono_tec.py
  • legacy/tropo_pyaps.py
  • solid_earth_tides.py
  • tropo_gacos.py
  • tropo_pyaps3.py

• cli/geocode.py: if --lat-file is specified, honor this manually set file as the default lookupFile, rather than searching for the default file patterns, which could be NOT consistent with input files.

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