Add flag useFixedOrder to the coarsen method of AMGs ParallelIndicesCoarsener.

If set to true, the indices for the coarser grid will be created in a fixed order, making parallel runs reproducible but the runtime is possibly not ideal.
If set to false (which is the default), the order depends on the order of messages recieved from the processes responsible for the respective parts of the finer grid.
Then the indices on the coarser grid may differ from run to run.