Use makeCluster() from parallel package instead of another package such ...

...as snow.

The following command finds masked functions between snow and parallel.
```grep -P "(clusterApply|clusterApplyLB|clusterCall|clusterEvalQ|clusterExport|clusterMap|clusterSplit|makeCluster|parApply|parCapply|parLapply|parRapply|parSapply|splitIndices|stopCluster)" R/*R```