Poisson-approval

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* In ``Profile``:

* Add ``d_candidate_plurality_welfare``: plurality welfare of each candidate.
* Add ``d_candidate_anti_plurality_welfare``: anti-plurality welfare of each candidate.
* Add ``d_candidate_relative_plurality_welfare``: relative plurality welfare of each candidate.
* Add ``d_candidate_relative_anti_plurality_welfare``: relative anti-plurality welfare of each candidate.

* In ``ProfileCardinal``:

* ``d_candidate_welfare`` and ``d_candidate_relative_welfare`` now return a ``DictPrintingInOrder`` instead of
a basic ``dict``.

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* This patch concerns only Poisson Approval's developpers. To develop and maintain the package, it uses GitHub actions
instead of additional services such as Travis-CI and ReadTheDocs.

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* Add ``plot_distribution_scores``: CDF of the score of the winner, the challenger and the loser (conditionally
on the convergence of fictitious play / iterated voting).
* Add ``TernaryAxesSubplotPoisson.f_point_values_``: when a candidate heatmap has been drawn, this function gives
access to the computed values.
* Add ``TauVector.print_magnitudes_order``: print the order of the magnitudes of the weak pivots.
* In ``fictitious_play`` and ``iterated_voting``, in verbose mode, display also the order of the magnitudes of
the weak pivots.

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* In ``TauVector``:

* Add ``share_single_votes``: share of single votes, i.e. votes for one candidate only.
* Add ``share_double_votes``: share of double votes, i.e. votes for two candidates.

* In ``ProfileCardinal``:

* Add ``share_sincere_among_strategic_voters``: share of strategic voters that happen to cast a sincere ballot (when
a strategy is given).
* Add ``share_sincere_among_fanatic_voters``: share of fanatic voters that happen to cast a sincere ballot.
* Add ``share_sincere``: share of voters that happen to cast a sincere ballot (when a strategy is given). This
takes sincere, fanatic and strategic voters into account.

* In ``Strategy``:

* Add ``share_single_votes`` and ``share_double_votes``: these shortcuts are defined when the strategy
is defined with an embedded profile.
* Add ``share_sincere_among_strategic_voters`` and ``share_sincere``: these shortcuts are defined when the strategy
is defined with an embedded profile, provided the profile is cardinal.

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* ``ProfileCardinal`` now has attributes ``d_candidate_welfare`` and ``d_candidate_relative_welfare``: for each
candidate, it gives its welfare, i.e. its total utility. The relative welfare is normalized so that the candidate
with maximal welfare has 1 and the one with minimal welfare has 0.
* The function ``probability`` now accepts a tuple of tests as inputs.
* Bug fix: the recent versions of the external package ``scipy`` changed the behavior of ``scipy.optimize.minimize``.
Since ``PivotTrio`` relies on this function, its behavior changed in an unexpected way and it sometimes lead to
incorrect results, such as a positive magnitude. This version solves the problem: ``PivotTrio`` has regained its
former (correct) behavior.

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* Add ``ternary_plot_convergence`` and ``binary_plot_convergence``: plot the convergence frequency, which is defined
as the proportion of initializations where iterated voting or fictitious play lead to convergence within
``n_max_episodes`` iterations.

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