Cpmpy

* fix bug in ort limitation check for 'modulo' operator
* mzn: better doc and check on single solution output
* various documentation updates

Major:
* re-enabled MiniZinc as a backend solver!
* reworked how solvers (and subsolvers) are accessed
-> you can now do `model.solve(solver="minizinc:chuffed")` and the like
* added a SolverLookup.solvernames() to get supported names
* a debugging guide in the docs

Minor:
* various documentation and test updates
* some more explicit errors
* add vectorized operations that were missing (thanks Hakan)
* pysat: fix bug where constraints were duplicated
* ort: show validation error when model is invalid
* ort: work around 'xor' not being reifiable
* add missing negated_normal for 'xor'

* make progress logging work in jupyter/IPython (beta ortools feature)
* transf/get_variables now has print_variables that prints domains, for debugging with domains
* fix automatic bounds computation of auxiliary variables for abs,mul,div,pow (mostly due to negative numbers)

* pysat: tseitin encode all logical operators
* to_cnf tseitin encoding for logical operators, with tests
* better chaining of n-ary operators, fixes 39
* doc: beginner tutorial add optimisation
* doc: extend multiple solutions, minisearch, diverse solutions
* ort: add OrtSolutionPrinter and solve(solution_callback=...)
* example of diverse solutions
* vectorized 'abs' operator
* flatten: fix some bound computations

* easier hyperparameter search with `param_combinations()` helper function in cpmpy.solvers

First beta release!

* Reorganize cpmpy/ modules (not backward-compatible)
* Rework variables/constraint constructors (deprecation warnings for old constructors, will be removed with stable release)
* Updated all examples to follow new style
* Add PySAT backend, with incrementality/core extraction (only accepts CNF input for now)
* Add minimize()/maximize() to Model() and solver objects
* simpler `from cpmpy.solvers import CPM_ortools` for solver-specific use
* Add keyword arguments to solve() that configure solver-specific options
* Add example of hyperparameter gridsearch
* Updated API and user docs considerably