Go faster if there is a single date

Avoid calling hunt in that case.

Common/read_snapshot.f90

@@ -39,7 +39,11 @@ contains
 
     !---------------------------------------------------------------------
 
-    call hunt([hshp%d0, huge(0)], k, i_shpc)
+    if (size(hshp) == 1) then
+       i_shpc = 1
+    else
+       call hunt([hshp%d0, huge(0)], k, i_shpc)
+    end if
 
     if (k == hshp(i_shpc)%d0) then
        ishape_first = 0