Fixed compilation error on Grendel

boutmesh.cxx(127): error: more than one instance of overloaded function "sqrt" matches the argument list:
            function "sqrt(double)"
            function "std::sqrt(long double)"
            function "std::sqrt(float)"
            argument types are: (int)
      BoutReal ideal = sqrt(  MX * NPES / ny); // Results in square domains

Added type cast