maximum and minimum: (ne,k) fails #34

ACE fails on the constraints maximum and minimum with the condition (ne,k), k being an integer.

<instance format="XCSP3" type="CSP">
  <variables>
    <array id="x" size="[3]"> 0..3 </array>
  </variables>
  <constraints>
    <maximum>
      <list> x[] </list>
      <condition> (ne,2) </condition>
    </maximum>
  </constraints>
</instance>

The expected solutions are the 45 tuples of 0..3³ whose maximum is not 2.

The same with <minimum> (57 expected solutions). With -ev:

java.lang.AssertionError: NE is not implemented
	at constraints.global.Extremum$ExtremumCst.buildFrom(Extremum.java:579)
	at problem.Problem.extremum(Problem.java:3152)
	at problem.Problem.maximum(Problem.java:3210)

Cause: ExtremumCst.buildFrom() handles LT, LE, GE, GT and EQ, and throws AssertionError("NE is not implemented") otherwise (// TODO useful to have a propagator?), whereas (ne,y), y being a variable, is accepted: in Problem.extremum(), a condition with a variable other than EQ goes to the generic case at the end of the method (simplifyComplexCondition()). The condition (ne,k) could be handled in the same way, or refused with a message saying that it is not implemented.

This issue was found by the unit tests of PyCSP3: xcsp3team/pycsp3#243