Datetime printing

[ewenmaclean]

this currently ipads to 3 correctly but doesn't cut off at 3 - e.g. with the millisec field as Some 5838 it prints .5838 rather than .584....

https://github.com/AestheticIntegration/fix-engine/blob/master/src-core-pp/encode_datetime.iml#L41

this is ugly but works - @KKostya what do you think?

let encode_UTCTimeOnly_milli (x:fix_utctimeonly_milli ) : string = 
    let hms = Printf.sprintf "%02u:%02u:%02u" 
                (Z.to_int x.utc_timeonly_hour)
                (Z.to_int x.utc_timeonly_minute)
                (Z.to_int x.utc_timeonly_second)
    in
    match x.utc_timeonly_millisec with
        | None -> hms 
        | Some ms -> hms 
          ^ (String.sub (Printf.sprintf ".%3d" (Z.to_int ms)) 0 4 |> 
            function 
            | None -> "" 
            | Some s -> s)
;;