text: Title: Drop milliseconds from plot titles
  Body: Time with milliseconds is a bit overkill on plot titles, and takes extra space too. I propose these changes to drop milliseconds from plot titles:
  
  `return num2date(times, units, calendar)` by `return num2date(times, units, calendar).replace(microsecond=0)` in functions `generate_radar_time_begin` (l126) and `generate_grid_time_begin` (l137) in `pyart/graph/common.py`