File access only through executor

[uellue]

Closes #965

Contributor Checklist:

• I have added or updated my entry in the creators.json file
• I have added a changelog entry for my contribution
• I have added/updated documentation for all user-facing changes
• I have added/updated test cases

Reviewer Checklist:

• /azp run libertem.libertem-data passed

[uellue]

/azp run libertem.libertem-data

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[codecov]

Codecov Report

Merging #966 (ad99165) into master (5059d84) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #966      +/-   ##
==========================================
+ Coverage   69.44%   69.46%   +0.01%     
==========================================
  Files         269      269              
  Lines       12080    12080              
  Branches     1643     1643              
==========================================
+ Hits         8389     8391       +2     
+ Misses       3385     3384       -1     
+ Partials      306      305       -1     

Impacted Files	Coverage Δ
src/libertem/io/dataset/empad.py	84.32% <ø> (ø)
src/libertem/io/dataset/frms6.py	72.36% <ø> (ø)
src/libertem/corrections/corrset.py	95.96% <0.00%> (+1.61%)	⬆️

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[sk1p]

LGTM!

[sk1p]

CI failure looks like sphinx-doc/sphinx#8880

[sk1p]

Possible workaround: pin sphinx to <3.5

[uellue]

Possible workaround: pin sphinx to <3.5

Ok, I'll try to push here. :-)

[sk1p]

One of the windows jobs again hit #963. Good thing is that with Azure Pipelines, we can restart failed jobs, and unlike travis, this also continues running further stages. So if we hit this again while releasing, we can re-trigger without problems (and without having to bump the version...)

[sk1p]

/azp run libertem.libertem-data

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).