fix output format for early GPS weeks / unix timstamps

GPSW.D integer part is left-padded, which leads to gaps in output for GPSw <1000:

branan@gfz:~/gnss-tools/dateconv$ bin/dateconv convert 0900.0 --format gpswd
0900.0
   -----------------------------------------------------
   | YYYY mm DD  DDD  MJD    GPSW.D  Second  Weekday   |
   -----------------------------------------------------
>> | 1997 04 06  096  50544  900 .0  0       Sunday    |
   | 1997 04 07  097  50545  900 .1  86400   Monday    |
   | 1997 04 08  098  50546  900 .2  172800  Tuesday   |
   | 1997 04 09  099  50547  900 .3  259200  Wednesday |
   | 1997 04 10  100  50548  900 .4  345600  Thursday  |
   | 1997 04 11  101  50549  900 .5  432000  Friday    |
   | 1997 04 12  102  50550  900 .6  518400  Saturday  |
   -----------------------------------------------------

   -----------------------------------------------------
   | HH MM SS SSSSSS  Unix        MJDF                 |
   -----------------------------------------------------
   | 00 00 00 000000  860284800  50544.00000          |
   -----------------------------------------------------

Please either make it right-padded or fill with leading zero!

Also, the format of the last line is off for unix timestamps < 1000000000.