readfile: add auto_no_data_value() for known files

[yunjunz]

Description of proposed changes

• utils.readfile: add auto_no_data_value() for know file types: dense offsets from isce2/topsApp.py

• utils.readfile: set the following functions as private, to discourage usage out of this sub-module:

  • _get_file_base_and_ext()
  • _sort_dataset_list2velocity()
  • _attribute_gamma2roipac()
  • _attribute_gmtsar2roipac()

• view.read_data4figure(): do not print out msg while reading a list of 2D matrix (fine tune for view.py fails when using large multilook factor #950).

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