Structure download section: add extxyz and xsf formats

[yakutovicha]

fixes #500
fixes #251

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@9a1ed90). Click here to learn what that means.
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #501   +/-   ##
=========================================
  Coverage          ?   79.69%           
=========================================
  Files             ?       27           
  Lines             ?     3757           
  Branches          ?        0           
=========================================
  Hits              ?     2994           
  Misses            ?      763           
  Partials          ?        0           

Flag	Coverage Δ
python-3.10	79.69% <0.00%> (?)
python-3.8	79.72% <0.00%> (?)
python-3.9	79.72% <0.00%> (?)

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