fix the dets order issue (#43)

This fixes the fallback logic for the geometric ordering (when no
coordinates are present). originally we used emplace_back on a pre-sized
vector and std::iota with incorrect iterators, resulting in the detector
orders not being filled correctly.
this solves those errors with : 
```
tesseract: src/tesseract.cc:106: TesseractDecoder::TesseractDecoder(TesseractConfig): Assertion `config.det_orders[i].size() == config.dem.count_detectors()' failed.
```

Author: LalehB

src/tesseract_main.cc

@@ -231,12 +231,13 @@ struct Args {
         std::vector<double> inner_products(config.dem.count_detectors());
 
         if (!detector_coords.size() || !detector_coords.at(0).size()) {
-          // If there are no detector coordinates, just use the standard ordering
-          // of the indices.
+          // If there are no detector coordinates, just use the standard
+          // ordering of the indices.
           for (size_t det_order = 0; det_order < num_det_orders; ++det_order) {
-            config.det_orders.emplace_back();
-            std::iota(config.det_orders.back().begin(), config.det_orders.front().end(), 0);
+            config.det_orders[det_order].resize(config.dem.count_detectors());
+            std::iota(config.det_orders[det_order].begin(), config.det_orders[det_order].end(), 0);
           }
+
         } else {
           // Use the coordinates to order the detectors based on a random
           // orientation
@@ -694,4 +695,4 @@ int main(int argc, char* argv[]) {
     std::cout << " total_time_seconds = " << total_time_seconds;
     std::cout << std::endl;
   }
-}
+}