wcst_import try to open 1 gdal dataset from input files instead of only the first 1

[Bang Pham Huu]

If the first file cannot be opened after validating phase, wcst_import will throw an exception when it cannot get some common metadata from the first file (e.g: null value). It should try with the next file instead.

  File "/rasv1/opt_rasdaman/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/abstract_to_coverage_converter.py", line 304, in _range_fields
    range_nils = self._get_nil_values(i)
  File "/rasv1/opt_rasdaman/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/abstract_to_coverage_converter.py", line 269, in _get_nil_values
    nil_values = self._file_band_nil_values(index)
  File "/rasv1/opt_rasdaman/rasdaman/share/rasdaman/wcst_import/recipes/general_coverage/gdal_to_coverage_converter.py", line 105, in _file_band_nil_values
    gdal_dataset = gdal.Open(self.files[0].filepath)
  File "/usr/lib/python2.7/dist-packages/osgeo/gdal.py", line 1800, in Open
    return _gdal.Open(*args)

[Dimitar Misev]

But note: only if skip is true.