[FIX] Fix opening GSF files in Multifile

[markotoplak]

The problem was actually a misbehaving reader that did not return the correct types.

The problem:

Traceback (most recent call last):
  File "/home/marko/dev/orange-spectroscopy/orangecontrib/spectroscopy/widgets/owmultifile.py", line 415, in browse_files
    self.load_files(filenames, reader)
  File "/home/marko/dev/orange-spectroscopy/orangecontrib/spectroscopy/widgets/owmultifile.py", line 426, in load_files
    self.load_data()
  File "/home/marko/dev/orange-spectroscopy/orangecontrib/spectroscopy/widgets/owmultifile.py", line 487, in load_data
    data = concatenate_data(data_list, fnok_list, self.label)
  File "/home/marko/dev/orange-spectroscopy/orangecontrib/spectroscopy/widgets/owmultifile.py", line 98, in concatenate_data
    xs = reduce(numpy_union_keep_order, xss, np.array([]))
  File "/home/marko/dev/orange-spectroscopy/orangecontrib/spectroscopy/widgets/owmultifile.py", line 40, in numpy_union_keep_order
    to_add = B[sorted(orig_sind[indices])]
TypeError: list indices must be integers or slices, not list

[codecov[bot]]

Codecov Report

Merging #677 (87a4986) into master (8b4f190) will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #677      +/-   ##
==========================================
- Coverage   89.38%   89.37%   -0.01%     
==========================================
  Files          72       72              
  Lines       11994    11994              
==========================================
- Hits        10721    10720       -1     
- Misses       1273     1274       +1