Try moving the decomposition of the MiniZinc gecode_cumulatives constraint into MiniZinc #81

The gecode_cumulatives MiniZinc constraint is used for implementing fzn_cumulative. The registry post function does a lot of inspection of the arguments, posting different variants of the constraint.

This requires first fixing #80 so that the relevant Gecode scheduling constraints are available in MiniZinc.

One potential benefit of this, is that the required start[i] + duration[i] = end[i] constraints would be available for common subexpression elimination for MiniZinc.