Poisson-approval

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* Add ``ProfileDiscrete``: a profile with a discrete distribution of voters.
* Subclasses of ``Profile``: better handling of the additional parameters like ``well_informed_voters`` or
``ratio_sincere``. In the conversions to string (``str`` or ``repr``), they are now mentioned. They are also used in
the equality tests between two profiles.

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* Implement ``ProfileCardinal.fictitious_play``, where the update ratios of the perceived tau-vector and the actual
tau-vector can be functions of the time. It is also faster that ``ProfileCardinal.iterated_voting``, but can
not detect cycles (only convergence).
* ``ProfileCardinal.iterated_voting_taus`` is renamed to ``ProfileCardinal.iterated_voting``. It has been generalized
by implementing a notion of perceived tau-vector, like for ``ProfileCardinal.fictitious_play``. The syntax has been
modified in consequence.
* ``ProfileCardinal.iterated_voting_strategies`` is deprecated and suppressed.
* Iterated voting and fictitious play do not need a ``StrategyThreshold`` as initial strategy, but any strategy that is
consistent with the profile subclass. For example, with ``ProfileTwelve``, you can use a ``StrategyTwelve``.
* ``Strategy.profile`` is now a property that can be reassigned after the creation of the object.
* Add ``Strategy.deepcopy_with_attached_profile``: make a deep copy and attach a given profile.
* Add the utility ``to_callable``: convert an object to a callable (making it a constant function if it is not
callable already).

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* Configure Codecov.
* Reach 100% coverage for this version.

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* In iterated voting, implement the possibility to move only *progressively* towards the best response:

* Add ``ProfileCardinal.iterated_voting_taus``: at each iteration, a given ratio of voters update their ballot.
* Replace the former method ``ProfileCardinal.iterated_voting`` by ``ProfileCardinal.iterated_voting_strategies``:
as in former versions, at each iteration, the threshold utility of each ranking's strategy is moved in the
direction of the best response's threshold utility. The method now returns a cycle of tau-vectors and the
corresponding cycle of best response strategies, in order to be consistent with
``ProfileCardinal.iterated_voting_taus``.
* Add the utility ``barycenter``: compute a barycenter while respecting the type of one input if the other input has
weight 0.
* Accelerate the algorithm used in iterated voting.

* In ``ProfileCardinal``, add the possibility of partial sincere voting:

* Add parameter ``ratio_sincere``: ratio of sincere voters.
* Add property ``tau_sincere``: the tau-vector if all voters vote sincerely.
* The former method ``tau`` is renamed ``tau_strategic``: the tau_vector if all voters vote strategically.
* The new method ``tau`` takes both sincere and strategic voting into account.
* The method ``is_equilibrium`` has a new implementation to take this feature into account.

* Add ``TauVector.isclose``: whether the tau-vector is close to another tau-vector (in the sense of
``math.isclose``). This method is used by the new version of ``ProfileCardinal.is_equilibrium``.

* Add ``Profile.best_responses_to_strategy``: convert a dictionary of best responses to a ``StrategyThreshold`` that
mentions only the rankings that are present in the profile.

* In random generators of profiles (``GeneratorProfileOrdinalUniform``, ``GeneratorProfileOrdinalGridUniform``,
``GeneratorProfileOrdinalVariations``, ``GeneratorProfileHistogramUniform``): instead of having explicit arguments
like ``well_informed_voters`` or ``ratio_sincere``, there are ``**kwargs`` that are directly passed to the
``__init__`` of the relevant Profile subclass.

* Update the tutorials with these new features.

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* Add ``image_distribution``: estimate the distribution of ``f(something)`` for a random ``something``.
* Update the tutorial on mass simulations with this new feature.

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* Add new random generators:

* ``GeneratorExamples``: run another generator until the generated object meets a given test.
* ``GeneratorStrategyOrdinalUniform``: draw a StrategyOrdinal uniformly.
* ``GeneratorProfileOrdinalGridUniform``: draw a ProfileOrdinal uniformly on a grid of rational numbers.
* ``GeneratorTauVectorGridUniform``: draw a TauVector uniformly on a grid of rational numbers.

* Utilities:

* Add ``rand_integers_fixed_sum``: draw an array of integers with a given sum.
* Add ``rand_simplex_grid``: draw a random point in the simplex, with rational coordinates of a given denominator.
* Update ``probability``: allow for a tuple of generators.

* Tutorials:

* Add a tutorial on asymptotic developments.
* Update the tutorial on mass simulations with the new features.

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