Add a __dataframe__ method to _VaexDataFrame

This will align the implementation with those in other libraries,
xref data-apis/dataframe-api#80

packages/vaex-core/vaex/dataframe_protocol.py

@@ -10,7 +10,7 @@
 For a background and spec, see:
    * https://data-apis.org/blog/dataframe_protocol_rfc/
    * https://data-apis.org/dataframe-protocol/latest/index.html
-                 
+
 Notes
 -----
 - Interpreting a raw pointer (as in ``Buffer.ptr``) is annoying and unsafe to
@@ -701,6 +701,9 @@ def __init__(self, df: vaex.dataframe.DataFrame, nan_as_null: bool = False, allo
         self._nan_as_null = nan_as_null
         self._allow_copy = allow_copy
 
+    def __dataframe__(self, nan_as_null : bool = False, allow_copy : bool = True) -> '_VaexDataFrame':
+        return _VaexDataFrame(self._df, nan_as_null=nan_as_null, allow_copy=allow_copy)
+
     @property
     def metadata(self) -> Dict[str, Any]:
         return {}

tests/dataframe_protocol_test.py

@@ -294,6 +294,8 @@ def test_VaexDataFrame():
     assert df2.select_columns((0, 2))._df[:, 0].tolist() == df2.select_columns_by_name(("x", "z"))._df[:, 0].tolist()
     assert df2.select_columns((0, 2))._df[:, 1].tolist() == df2.select_columns_by_name(("x", "z"))._df[:, 1].tolist()
 
+    assert_dataframe_equal(df2, df2.__dataframe__())
+
 
 def test_chunks(df_factory):
     x = np.arange(10)