DOC Fix 3d example failing import

[MuawizChaudhary]

The 3D example on dev fails an import due to the fact that we now use a class structure for the backend. This PR fixes this by importing instead the backend and calling compute integrals.

Currently in the process of running the full example on a google colab.

This goes a small way of solving #817 .

[codecov-commenter]

Codecov Report

Merging #829 (1f38623) into dev (ca9073f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #829   +/-   ##
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  Coverage   88.03%   88.03%           
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  Files          64       64           
  Lines        2323     2323           
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  Hits         2045     2045           
  Misses        278      278           

Flag	Coverage Δ
jenkins_main	88.03% <ø> (ø)

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[edouardoyallon]

looks good

[MuawizChaudhary]

The google colab has finished running. @eickenberg do the results look reasonable to you?

[janden]

Thanks!

[MuawizChaudhary]

Mind if we merge?