FAIR module: Two copies of AR6 parameters in `fair_temperature_fit.py`

[e-marshall]

In the fair_fit_temperature() function (located in fair_temperature_fit.py), param_file is an input argument with a netcdf specified as the default. Within the function, the same netcdf file is read into memory as a separate object (pars). Both objects are combined in a dictionary and written as a pickle file that is used in fair_temperature_project.py. but, it looks like the instance where param_file is used in this script could be replaced with pars (line 190), or vice versa. Could we remove the hard-coded object and use only the parameter object passed as an input argument?