Run constraint testsuite on all solvers

[IgnaceBleukx]

When the Gurobi solver was added to the CPMpy library, tests for all flattened constraints were added in /tests/test_constraints.py.

Still to be done is running this suite on OR-tools and Minizinc solvers (and Pysat when int-to-bool is implemented)

[IgnaceBleukx]

Minizinc:

• The division operator is now implemented as the float division in minizinc (/). Should this be changed to integer division? (related: a / 2 != b is not supported by Gecode, a div 2 != b is),
• Modulo with negative arguments is buggy at the moment (bug in minizinc itself:https://github.com/MiniZinc/libminizinc/issues/566)
• Power operator in implications are buggy: p == (pow(a,2) >=< b) gives a compiler error in FlatZinc: fixed in https://github.com/MiniZinc/libminizinc/issues/572
• Circuit constraint does not equal with our decomposition of circuit: solution by minizinc: [2,3,1] while in the decomposition of circuit, the array must end with 0

OR-Tools:

• https://github.com/CPMpy/cpmpy/issues/105

Pysat:

• any numeric constraints... int2bool branch

Gurobi:

• Many implication constraints not supported. Some of which can be solved using the same trick as in #105 (some_unsupported_operator >=< a) == b becomes (some_unsupported_operator == c & (c >=< a) == b) Implementation should be done in linearize transformation. Or make use of the soon to be transformation from #105

Solver agnostic:

• Implement value() for circuit and table constraints

[Wout4]

we only support integer division (//), and the test_constraints test-suite is now ran on all supported solvers