adds explicit type conversion to avoid warning

cpp/particles/p2p_ghost_collision_base.hpp

@@ -44,7 +44,7 @@ template < class partsize_t>
 std::set <std::pair <partsize_t,partsize_t>> vec2pairs(std::vector<partsize_t> v){
   std::set <std::pair <partsize_t,partsize_t>> ID_pairs;
   assert(v.size()%2 == 0);
-  for(unsigned int i=0; i < v.size()/2; i++)
+  for(int i=0; i < int(v.size())/2; i++)
     {
       //DEBUG_MSG((std::to_string(v[2*i])+" "+std::to_string(v[2*i+1])+"\n").c_str());
       std::pair <partsize_t, partsize_t> single_collision_pair(v[2*i],v[2*i+1]);
@@ -57,7 +57,7 @@ template <class partsize_t>
 void print_pair_vec(std::vector<partsize_t> vec)
 {
     assert(vec.size() % 2 == 0);
-    for (int i = 0; i< vec.size()/2; i++)
+    for (int i = 0; i < int(vec.size())/2; i++)
         DEBUG_MSG("in print_pair_vec, pair %d is (%ld,%ld)\n", i, vec[2*i], vec[2*i+1]);
 }