Expose sorting variable argument in the command line interface

[danielfromearth]

As discussed with @cheyenneland, time_variable is an argument to stitchee() and is used for determining the file order before concatenation, but time_variable is not yet exposed in the CLI defined in run_stitchee.py. Being able to set time_variable when running stitchee from the command line will allow setting it to the appropriate value for each mission's data.

[danielfromearth]

@cheyenneland, this has been merged into the develop branch, with a new name of sorting_variable and a default to not sort the input datasets. So I think this is ready to test with CERES data.