Dedup DFT grid keyword the same way as SCF/DLPNO

The DFT branch unconditionally appended defgrid2/defgrid3 via
add_to_args, so a user who set their own defgrid keyword for a DFT
sp_level got it twice in the ! line. Apply the same setdefault pattern
we just used for scf_convergence and dlpno_threshold under the key
'dft_grid', so the default only lands when the user hasn't already
supplied one.

Author: calvinp0

arc/job/adapters/orca.py

@@ -290,11 +290,9 @@ def write_input_file(self) -> None:
         # Orca requires different blocks for wavefunction methods and DFT methods
         if self.level.method_type == 'dft':
             input_dict['method_class'] = 'KS'
-            # DFT grid must be the same for both opt and freq
-            if self.fine:
-                self.add_to_args(val='defgrid3', key1='keyword')
-            else:
-                self.add_to_args(val='defgrid2', key1='keyword')
+            # DFT grid must be the same for both opt and freq.
+            # Users can override by setting `dft_grid` in args.keyword (e.g. dft_grid: DEFGRID1).
+            self.args['keyword'].setdefault('dft_grid', 'defgrid3' if self.fine else 'defgrid2')
         elif self.level.method_type == 'wavefunction':
             input_dict['method_class'] = 'HF'
             if 'dlpno' in self.level.method: