fix: fix dtype of keywords_high_multiplicity (#1582)

which should have the same `dtype` as `keywords`.

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-->

## Summary by CodeRabbit

- **Improvements**
- Enhanced flexibility in configuration by allowing
`keywords_high_multiplicity` to accept both strings and lists of
strings.

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Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

dpgen/generator/arginfo.py

@@ -709,7 +709,7 @@ def fp_style_gaussian_args() -> list[Argument]:
         Argument("basis_set", str, optional=True, doc=doc_basis_set),
         Argument(
             "keywords_high_multiplicity",
-            str,
+            [str, list[str]],
             optional=True,
             doc=doc_keywords_high_multiplicity,
         ),