Set a default binning policy in mc_cell_group (#384)

Fixes #383 (segfault owing to empty vector of binners).

* Add call to `set_binning_policy` in `mc_cell_group` constructor with default `binning_kind::none` policy.

src/mc_cell_group.hpp

@@ -40,6 +40,9 @@ public:
     mc_cell_group(std::vector<cell_gid_type> gids, const recipe& rec):
         gids_(std::move(gids))
     {
+        // Default to no binning of events
+        set_binning_policy(binning_kind::none, 0);
+
         // Build lookup table for gid to local index.
         for (auto i: util::make_span(0, gids_.size())) {
             gid_index_map_[gids_[i]] = i;