Update driver

 - Use new linear regression function `calc_linear_regression` which uses xskillscore and produces same results as old function
- Update variable names to denote xr.Dataset objects
- Update `_create_metrics_dict()` arguments
- Rename `calculate_nino_index()` to `calculate_nino_index_obs()`
- Extract `_get_contour_levels()` from `run_diags_map()`
- Add `_align_time_coords()` to align nino index time coords to variable time coords