time_type = double (#984)

* Change `time_type` to `double` to support time-step accuracy in longer running simulations.
* Change connection delay to `float` from `time_type` in order to avoid unnecessary memory overheads.

arbor/connection.hpp

@@ -36,8 +36,8 @@ public:
 private:
     cell_member_type source_;
     cell_member_type destination_;
+    float delay_;
     float weight_;
-    time_type delay_;
     cell_size_type index_on_domain_;
 };
 

arbor/include/arbor/common_types.hpp

@@ -54,7 +54,7 @@ ARB_DEFINE_LEXICOGRAPHIC_ORDERING(cell_member_type,(a.gid,a.index),(b.gid,b.inde
 
 // For storing time values [ms]
 
-using time_type = float;
+using time_type = double;
 constexpr time_type terminal_time = std::numeric_limits<time_type>::max();
 
 // Extra contextual information associated with a probe.