Groupby operations on dataframes (#3473)

* Added PolarsStruct enum to implement groupby

* template groupby

* groupby operationi on dataframes

crates/nu-value-ext/src/lib.rs

@@ -725,7 +725,7 @@ pub fn get_data<'value>(value: &'value Value, desc: &str) -> MaybeOwned<'value,
             MaybeOwned::Owned(UntaggedValue::nothing().into_untagged_value())
         }
         #[cfg(feature = "dataframe")]
-        UntaggedValue::Dataframe(_) => {
+        UntaggedValue::DataFrame(_) => {
             MaybeOwned::Owned(UntaggedValue::nothing().into_untagged_value())
         }
     }