possibility for infinite loop in only_bv_implies

[Wout4]

Only_bv_implies goes into a loop when a reified global is given as input.
For now this never occurs in the wild since no solvers using only_bv_implies support any reified globals, but that could be possible in the future. Otherwise we should clearly state this precondition for only_bv_implies.

example:
bv = boolvar(shape=3)
only_bv_implies([Xor([bv[0],bv[1]]) == bv[2]])