Investigate if rewriting to `cumulatives` is good for zero-length tasks when posting MiniZinc constraints `gecode_cumulatives`

[zayenz]

In the registry function for gecode_cumulatives the case where the tasks may have zero-length duration and the bound is not assigned is currently decomposed into basic constraints.

An alternative re-write strategy would be to rewrite to cumulatives using multiple machines, where zero-length tasks are on a different machine with an "unlimited" bound.

If #81 has been implemented, this should be tested in the new MiniZinc rewriting, otherwise in the current C++ code.