Add link to source code in the documentation

[andrea-pasquale]

I've added the sphinx extension viewcode which allows to visualize the source code directly from the documentation.

After a quick discussion with @BrunoLiegiBastonLiegi we think that it would be useful to improve the quality of the documentation.

If we like this approach I can propagate it to the other repos.

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c113117) 100.00% compared to head (d8a56d0) 100.00%.
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1195   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           69        69           
  Lines        10112     10112           
=========================================
  Hits         10112     10112           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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