outp: fix EveryNth output

This had worked, then I broke it, this should undo the breakage.

Author: germasch

src/libpsc/psc_output_particles/output_particles_hdf5_impl.hxx

@@ -445,7 +445,22 @@ struct OutputParticlesHdf5
     int nr_kinds = grid.kinds.size();
 
     // count all particles to be written locally
-    size_t n_write = mprts.size();
+    size_t n_write = 0;
+    for (int p = 0; p < mprts.n_patches(); p++) {
+      auto& patch = grid.patches[p];
+      int ilo[3], ihi[3], ld[3];
+      int sz = find_patch_bounds(grid.ldims, patch.off, ilo, ihi, ld);
+      for (int jz = ilo[2]; jz < ihi[2]; jz++) {
+        for (int jy = ilo[1]; jy < ihi[1]; jy++) {
+          for (int jx = ilo[0]; jx < ihi[0]; jx++) {
+            for (int kind = 0; kind < nr_kinds; kind++) {
+              int si = sort_index(grid.ldims, nr_kinds, {jx, jy, jz}, kind);
+              n_write += off[p][si + 1] - off[p][si];
+            }
+          }
+        }
+      }
+    }
 
     assert(sizeof(size_t) == sizeof(unsigned long));
     size_t n_total, n_off = 0;