Use MPI_RANK as ECL keyword (max length is 8)

MPI_RANKS was one character to long and rose an exception with message
"Error: Program threw an exception: Keyword is too long."

opm/autodiff/FlowMainEbos.hpp

@@ -824,7 +824,7 @@ namespace Opm
                 Handle handle(globalMapper, ranks,
                               this->globalGrid().globalCell());
                 this->grid().gatherData(handle);
-                integerVectors.emplace("MPI_RANKS", ranks);
+                integerVectors.emplace("MPI_RANK", ranks);
             }
 
             return integerVectors;