Improve speed of OI

Create Point objects for each observation point once at the beginning of
the function, so that they don't get created many times when passing
into corr().

This makes up for the lost time caused by 6f6a551, where there is extra
overhead dealing with the vector version of corr(). However, with this
fix, OI is even faster than v0.7.0 (about 5%).

src/api/oi.cpp

@@ -211,6 +211,13 @@ vec gridpp::optimal_interpolation_full(const gridpp::Points& bpoints,
     // Compute the background value at observation points (Y)
     vec gY = pbackground;
 
+    // Create all objects of type Point (to save time on creation later)
+    std::vector<Point> point_vec;
+    point_vec.reserve(nS);
+    for(int i = 0; i < nS; i++) {
+        point_vec.push_back(points.get_point(i));
+    }
+
     #pragma omp parallel for
     for(int y = 0; y < nY; y++) {
         if(!gridpp::is_valid(background[y])) {
@@ -238,8 +245,7 @@ vec gridpp::optimal_interpolation_full(const gridpp::Points& bpoints,
         p2.reserve(lLocIndices0.size());
         for(int i = 0; i < lLocIndices0.size(); i++) {
             int index = lLocIndices0[i];
-            Point p = points.get_point(index);
-            p2.push_back(p);
+            p2.push_back(point_vec[index]);
         }
         vec rhos = structure.corr_background(p1, p2);
         for(int i = 0; i < lLocIndices0.size(); i++) {
@@ -295,12 +301,11 @@ vec gridpp::optimal_interpolation_full(const gridpp::Points& bpoints,
             lY(i) = gY[index];
             lR(i, i) = pratios[index];
             lG(0, i) = lRhos(i);
-            Point p1 = points.get_point(index);
+            Point p1 = point_vec[index];
             std::vector<Point> p2(lS, Point(0, 0));
             for(int j = 0; j < lS; j++) {
                 int index_j = lLocIndices[j];
-                Point p = points.get_point(index_j);
-                p2[j] = p;
+                p2[j] = point_vec[index_j];
             }
             vec corr = structure.corr(p1, p2);
             for(int j = 0; j < lS; j++) {

src/api/oi_ensi.cpp

@@ -176,6 +176,13 @@ vec2 gridpp::optimal_interpolation_ensi(const gridpp::Points& bpoints,
         gYhat[i] = mean;
     }
 
+    // Create all objects of type Point (to save time on creation later)
+    std::vector<Point> point_vec;
+    point_vec.reserve(nS);
+    for(int i = 0; i < nS; i++) {
+        point_vec.push_back(points.get_point(i));
+    }
+
     // Calculate number of valid members
     int nValidEns = 0;
     ivec validEns;
@@ -219,8 +226,7 @@ vec2 gridpp::optimal_interpolation_ensi(const gridpp::Points& bpoints,
         p2.reserve(lLocIndices0.size());
         for(int i = 0; i < lLocIndices0.size(); i++) {
             int index = lLocIndices0[i];
-            Point p = points.get_point(index);
-            p2.push_back(p);
+            p2.push_back(point_vec[index]);
         }
         vec rhos = structure.corr_background(p1, p2);
         for(int i = 0; i < lLocIndices0.size(); i++) {