Use rounding for dispersers from cell (#223)

Replaced flooring by rounding in computation of number of dispersers from a cell.

include/pops/host_pool.hpp

@@ -307,8 +307,7 @@ class HostPool : public HostPoolInterface<RasterIndex>
             }
         }
         else {
-            dispersers_from_cell =
-                static_cast<int>(std::floor(lambda * infected_at(row, col)));
+            dispersers_from_cell = std::lround(lambda * infected_at(row, col));
         }
         return dispersers_from_cell;
     }