Fix missing cell_methods for instantaneous files from um2nc

[joshuatorrance]

• Added fix_cell_methods that will add time:point to the cell_methods of a variable if:
  • There is time
  • There are no time bounds (i.e. time has not bounds attribute)
  • The string time is not present in the current cell_methods
• The number of arguments to process_file was getting silly so this function now uses kwargs

Closes #7

[blimlim]

Thanks for adding this @joshuatorrance, this looks great!