fix factor in adjoint gradient from symmetry (#1527)

* slight clean up meep.i

* symfix

* use ivec

Co-authored-by: Mo Chen <mochen@MacBook-Pro-2.local>

src/meep/vec.hpp

@@ -1079,6 +1079,7 @@ class symmetry {
   int transform(int, int n) const;
   std::complex<double> phase_shift(int, int n) const;
   int multiplicity() const;
+  int multiplicity(ivec &) const;
   bool is_primitive(const ivec &) const;
 
   volume_list *reduce(const volume_list *gl) const;

src/near2far.cpp

@@ -660,6 +660,7 @@ std::vector<struct sourcedata> dft_near2far::near_sourcedata(const vec &x_0, dou
       sourcedata temp_struct = {component(f->c), idx_arr, f->fc->chunk_idx, amp_arr};
 
       LOOP_OVER_IVECS(f->fc->gv, f->is, f->ie, idx) {
+        IVEC_LOOP_ILOC(f->fc->gv, ix0);
         IVEC_LOOP_LOC(f->fc->gv, x0);
         x0 = f->S.transform(x0, f->sn) + rshift;
         vec xs(x0);
@@ -692,6 +693,7 @@ std::vector<struct sourcedata> dft_near2far::near_sourcedata(const vec &x_0, dou
         idx_dft++;
         if (is_electric(temp_struct.near_fd_comp))
           EH0 *= -1;
+        EH0 /= f->S.multiplicity(ix0);
         temp_struct.amp_arr.push_back(EH0);
       }
     }

src/vec.cpp

@@ -1200,6 +1200,15 @@ int symmetry::multiplicity() const {
     return g;
 }
 
+int symmetry::multiplicity(ivec &x) const {
+  int m = multiplicity();
+  for (int n=1; n<m; ++n){
+    if (transform(x,n) == x)
+      return n;
+  }
+  return m;
+}
+
 symmetry symmetry::operator+(const symmetry &b) const {
   // The following optimization ignores identity when adding symmetries
   // together.  This is important because identity has an undefined