Adding the possibility to either use MPI Allreduce or Reduce, depending on...
Adding the possibility to either use MPI Allreduce or Reduce, depending on whether or not the grids need to be available on all processes (Allreduce), or on the master process only (Reduce). I concluded that Allreduce must require more communication between processes than needed, possibly negatively impacting MPI efficiency. I only changed the function SnowpackInterface::getGrid(...). For the ebalance module, I'm not sure which grids need to be communicated using Allreduce vs Reduce.