generalize SARIXModel to take sources other than NHSN

[nickreich]

We would like the SARIX model to be able to be run on NSSP data. Currently the model is hard-coded to expect only NHSN data, as is visible here:

idmodels/src/idmodels/sarix.py

Line 17 in 154728b

df = fdl.load_data(nhsn_kwargs={"as_of": run_config.ref_date, "disease": run_config.disease},

We'd like to generalize