Fix validity time attributes

[blimlim]

Closes #28. This PR swaps the change_dump_date.py script to set the validity time using the t2_(year/month/day) attributes rather than v1_(year/month/day), which don't exist in the mule fixed_length_header` attribute.

Using the updated script to set the date to 18500101 in an ESM1.6 restart, we see that the initial data time and validity times are correctly set:

  (21) t1_year                          :      1850
  (22) t1_month                         :         1
  (23) t1_day                           :         1
  (24) t1_hour                          :         0
  (25) t1_minute                        :         0
  (26) t1_second                        :         0
  (27) t1_year_day_number               :         1
  (28) t2_year                          :      1850
  (29) t2_month                         :         1
  (30) t2_day                           :         1
  (31) t2_hour                          :         0
  (32) t2_minute                        :         0
  (33) t2_second                        :         0
  (34) t2_year_day_number               :         1

[atteggiani]

Thanks @blimlim for fixing this!

Looks good