r.latlong: return central pixel values for lat or long (#6611)

* Update main.c to return central pixel values for lat or long

Ref to issue #3274 from @mankoff

* Revert "Update main.c to return central pixel values for lat or long"

This reverts commit 20cbf59.

* Adjust lat and lon to cell center

* Update test

---------

Co-authored-by: Yann Chemin <[email protected]>

Author: mankoff

raster/r.latlong/main.c

@@ -125,8 +125,8 @@ int main(int argc, char *argv[])
         Rast_get_d_row(infd, inrast, row);
 
         for (col = 0; col < ncols; col++) {
-            latitude = ymax - ((double)row * stepy);
-            longitude = xmin + ((double)col * stepx);
+            latitude = ymax - ((double)row * stepy) - (stepy / 2.0);
+            longitude = xmin + ((double)col * stepx) + (stepx / 2.0);
             if (not_ll) {
                 if (GPJ_transform(&iproj, &oproj, &tproj, PJ_FWD, &longitude,
                                   &latitude, NULL) < 0)

raster/r.latlong/tests/test_r_latlong.py

@@ -73,9 +73,9 @@ def test_latlong_lat(setup_maps):
 west: 0
 rows: 3
 cols: 3
-3 3 3
-2 2 2
-1 1 1"""
+2.5 2.5 2.5
+1.5 1.5 1.5
+0.5 0.5 0.5"""
 
     assert ascii_clean == expected_output
 
@@ -105,9 +105,9 @@ def test_l_flag(setup_maps):
 west: 0
 rows: 3
 cols: 3
-0 1 2
-0 1 2
-0 1 2"""
+0.5 1.5 2.5
+0.5 1.5 2.5
+0.5 1.5 2.5"""
 
     assert ascii_clean == expected_output