Update API for CaseClass

This first commit is done in anticipation of supporting time series files in
addition to history files, and also using intake-esm catalogs behind the
scenes.

1. start_date & end_date are not part of the class constructor
   -- basically all the __init__ routine should do is set up a catalog
   -- _find_hist_files() also doesn't rely on these variables either; this
      routine is essentially building the catalog, and we don't restrict the
      files listed until we are ready to call open_mfdataset()
2. _open_history_files() -> gen_dataset(); this routine is where start_date and
   end_date are specified, and it returns a dataset rather than updating a
   class member variable. Logic to apply start_date and end_date at this stage
   is much simpler than doing it in _find_hist_files() (or I figured out a much
   easier way to do it). This routine also now expects a list of variable names
   to include in the dataset.
3. compare_fields_at_lat_lon expects array of DataArrays, not array of cases
4. Added get_varnames_from_metadata_list() to utils/utils.py because I need
   this function to generate the list of variable names for gen_dataset() in
   several notebooks.

I also created a script that submits jobs to the slurm queue to re-run
notebooks, though it does not work for the trend_maps or plot_suite_00[34]
notebooks (I think because of the reliance on dask and NCAR_jobqueue).

notebooks/logs/.gitignore

@@ -0,0 +1,2 @@
+*.err.*
+*.out.*