Introduce pip extras

[alecandido]

Closes #1251

They were obviously missing when the dependencies have been made optional.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.95%. Comparing base (d1fe5a9) to head (bb1a137).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1252   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          72       72           
  Lines       10454    10454           
=======================================
  Hits        10449    10449           
  Misses          5        5           

Flag	Coverage Δ
unittests	99.95% <ø> (ø)

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[stavros11]

Thanks @alecandido. I cannot tell anything about nix but confirmed that tensorflow is no longer installed by default in this branch.

[alecandido]

Currently, I'm the only one using Nix (though it's there for everyone, and you could have a reproducible shell).
However, the baseline is that I'm updating that as well, to avoid outdated content in our repo, but the core of the PR is just Poetry.