adds required method

cpp/field.hpp

@@ -208,6 +208,13 @@ class field
             return *(this->data + cindex*2 + imag);
         }
 
+        inline rnumber &cval(ptrdiff_t cindex, int imag) const
+        {
+            assert(fc == ONE);
+            assert(imag == 0 || imag == 1);
+            return *(this->data + cindex*2 + imag);
+        }
+
         inline rnumber &cval(ptrdiff_t cindex, int component, int imag)
         {
             assert(fc == THREE);