Assemble AD2CP timestamp with nanosecond precision (#1436)

* assemble ad2cp timestamp at ns instead of us

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* add noqa

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

echopype/convert/parse_ad2cp.py

@@ -328,7 +328,8 @@ def timestamp(self) -> np.datetime64:
         microsec100 = self.data["microsec100"]
         try:
             return np.datetime64(
-                f"{year:04}-{month:02}-{day:02}T{hour:02}:{minute:02}:{seconds:02}.{microsec100:04}"
+                f"{year:04}-{month:02}-{day:02}T{hour:02}:{minute:02}:{seconds:02}.{microsec100:04}",  # noqa
+                "ns",
             )  # type: ignore
         except ValueError:
             return np.datetime64("NaT")  # type: ignore