Revert date convention for MPASSIT output

[EricJames-NOAA]

Based on testing on Hera, we don't need to have timestr defined as 00:00:00.nc. We can revert to the native MPAS format of 00.00.00.nc to simplify things.

[guoqing-noaa]

This is good news! Thanks, @EricJames-NOAA