Expose Gecode scheduling constraints with (start, duration, end) tasks to MiniZinc

[zayenz]

In Gecode, many scheduling constraints constraints take a triple of start, duration, and end variables for each task, while the coresponding constraints in MiniZinc work with only a start and duration.

The idea is to add new custom constraints in gecode.mzn for the cumulative, cumulatives, and unary variants and matching posting functions in the registry.