coord/base: remove explicit from scalar constructor

we need to create vectors of zeros pretty often and it's pretty verbose
having to continually use the full type name each time.

coord/base.hpp

@@ -66,7 +66,7 @@ namespace util::coord {
         base () = default;
 
         /// constructs an instance where all elements are initialised to `val'.
-        constexpr explicit
+        constexpr
         base (T fill)
         {
             for (decltype(S) i = 0; i < S; ++i)