reserve() for wells_rep_radius and wells_perf_length

for more efficient push_back()

opm/polymer/fullyimplicit/SimulatorFullyImplicitBlackoilPolymer_impl.hpp

@@ -166,9 +166,14 @@ namespace Opm
             return;
         }
 
+        const size_t n_perf = wells_rep_radius.size();
+
         wells_rep_radius.clear();
         wells_perf_length.clear();
 
+        wells_rep_radius.reserve(n_perf);
+        wells_perf_length.reserve(n_perf);
+
         std::map<int,int> cartesian_to_compressed;
 
         setupCompressedToCartesian(global_cell, number_of_cells,