Various small fixes.

Not sure how this compiled in the other dev environment...
nonetheless, should be fixed.

miniapp/miniapp_recipes.cpp

@@ -56,7 +56,7 @@ public:
                             - param_.min_connection_delay_ms};
     }
 
-    cell_size_type num_cells() const { return ncell_; }
+    cell_size_type num_cells() const override { return ncell_; }
 
     cell get_cell(cell_gid_type i) const override {
         auto gen = std::mt19937(i); // replace this with hashing generator...

src/cell_group.hpp

@@ -52,7 +52,7 @@ public:
     void reset() {
         remove_samplers();
         initialize_cells();
-        for (auto& spike_source: spike_sources) {
+        for (auto& spike_source: spike_sources_) {
             spike_source.source.reset(cell_, 0.f);
         }
     }

src/util/ioutil.hpp

@@ -39,11 +39,11 @@ public:
     virtual ~basic_null_streambuf() {}
 
 protected:
-    virtual std::streamsize xsputn(const char_type *s,std::streamsize count) {
+    std::streamsize xsputn(const char_type *s,std::streamsize count) override {
         return count;
     }
 
-    virtual int_type overflow(char c) {
+    int_type overflow(int_type c) override {
         return traits_type::not_eof(c);
     }
 };