Avoid a copy on full vector replacement by using std::move.

When a queued update replaces an entire variable, we can re-use the
vector contained in the update by moving it over, instead of copying it.

This situation is actually pretty common in malariasimulation, where
exponentially decaying variables are entirely overwritten at each time
step. This change speeds up the simulation by about 5% for a 1M
population.

inst/include/vector_variables.h

@@ -22,9 +22,9 @@ inline void vector_update(
     std::vector<A>& values
     ) {
     while(updates.size() > 0) {
-        const auto& update = updates.front();
-        const auto& new_values = update.first;
-        const auto& index = update.second;
+        auto& update = updates.front();
+        auto& new_values = update.first;
+        auto& index = update.second;
 
         auto vector_replacement = (index.size() == 0);
         auto value_fill = (new_values.size() == 1);
@@ -34,7 +34,7 @@ inline void vector_update(
             if (value_fill) {
                 std::fill(values.begin(), values.end(), new_values[0]);
             } else {
-                values = new_values;
+                values = std::move(new_values);
             }
         } else {
             if (value_fill) {