Pymzn

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* Support parsing of saved solution streams.
* Bugfixes.

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* Massive redesign of the whole pipeline using features from MiniZinc 2.2.0
(earlier MiniZinc versions are no longer supported, yet PyMzn 0.17.1 should
work fine for the most part).
* Now PyMzn interfaces only to the `minizinc` executable, greatly
simplifying internal complexity.
* The `minizinc` function now only executes MiniZinc synchronously (i.e. wait
for it to finish before parsing the solution stream).
* Asyncronous solving is now handled via Python's `asyncio` package. The new
`pymzn.aio` module contains the `minizinc` *coroutine*, i.e. an asyncronous
version of the `pymzn.minizinc` function. The `pymzn.aio` module requires
Python >= 3.6.
* PyMzn can now parse MiniZinc enums into Python Enums and back.
* Substantial improvement of the preprocessing, solving and solution parsing.
* The `Solutions` class returned by the `minizinc` function has been improved
too.
* Removed the `MiniZincModel` class for dynamic modelling, just use Jinja
instead.
* Improved configuration facility.
* The `pymzn` command line executable has been greatly improved.
* This version is not backward compatible. Most of the existing code will need
to be adapted.

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* Introduced templating using Jinja2. The ``pymzn.MiniZincModel`` class can now
compile templated models, and that is also done automatically using
``pymzn.minizinc``.
* The ``pymzn.templates`` added for managing template search.
* Substantially improve the API for managing external processes. Now the
``pymzn.Process`` class takes care of executing either synchronously or
asynchronously an external process.
* The functions ``pymzn.minizinc`` and ``pymzn.solns2out`` have become
asynchronous by default.
* The ``pymzn.minizinc`` now returns a instance of ``pymzn.Solutions``, which
receives lazily the solutions from the solving pipeline, but can also be
referenced as a list after receiving the full solution stream.
* Add ``num_solutions`` parameter to ``pymzn.minizinc``.
* API changes:
* Deleted ``pymzn.utils`` module, along with the ``pymzn.run`` function
* Added ``pymzn.process`` module
* Renamed ``pymzn.SolnStream`` into ``pymzn.Solutions``
* The function ``pymzn.solns2out`` now returns a generator
* The function ``pymzn.minizinc`` is no longer blocking until the underlying
solving process is completed. It now returns a ``pymzn.Solutions`` that
can be lazily accessed.
* Added ``solve_start`` method to ``pymzn.Solver`` class which returns a
started process that produces solutions asynchronously.
* Several improvements and bug fixes.

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* Function ``pymzn.minizinc`` now returns a ``SolnStream``
* Completely revisited ``pymzn.Solver`` abstract class.
* Added support to many new solvers:
* Chuffed
* CBC
* Gurobi
* G12 (fd, lazy, mip)
* API changes:
* Renamed ``pymzn.dzn_eval`` into ``pymzn.dzn2dict``
* Renamed ``pymzn.dzn`` into ``pymzn.dict2dzn``
* Renamed ``pymzn.dzn_statement`` into ``pymzn.stmt2dzn``
* Renamed ``pymzn.dzn_value`` into ``pymzn.val2dzn``
* Renamed ``path`` into ``include`` in the ``pymzn.minizinc`` and
``pymzn.mzn2fzn`` functions.
* Introduced ``output_mode`` in place of ``parse_output`` and
``eval_output`` as parameter to the ``pymzn.minizinc`` function.
* Moved ``pymzn.run`` into utils
* Slim down the library quite a bit. Removed unnecessary stuff.
* Many bug fixes