Specify editor when loading file

[ericpre]

Fixes #37: specify the open file editor.

[codecov-commenter]

Codecov Report

Merging #38 (12df62a) into master (62fd667) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   57.11%   57.19%   +0.08%     
==========================================
  Files          16       16              
  Lines         506      507       +1     
  Branches       43       43              
==========================================
+ Hits          289      290       +1     
  Misses        210      210              
  Partials        7        7              

Impacted Files	Coverage Δ
hyperspy_gui_traitsui/tools.py	34.91% <100.00%> (+0.38%)	⬆️

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[ericpre]

Alternative would be to specify File(exists=True) in https://github.com/hyperspy/hyperspy/blob/ea89fe2a12609b7fe272e048a9bd83af8a51dfda/hyperspy/signal_tools.py#L748-L750

but this doesn't work - see https://github.com/enthought/traits/issues/1459.