Prevent delaunay triples from setting header for non-file output

Author: weiji14

pygmt/src/triangulate.py

@@ -355,6 +355,9 @@ def delaunay_triples(  # pylint: disable=too-many-arguments,too-many-locals
             warnings.warn(message=msg, category=RuntimeWarning, stacklevel=2)
             output_type = "file"
 
+        if header is not None and output_type != "file":
+            raise GMTInvalidInput("'header' is only allowed with output_type='file'.")
+
         # Return a pandas.DataFrame if ``outfile`` is not set
         with GMTTempFile(suffix=".txt") as tmpfile:
             if output_type != "file":

pygmt/tests/test_triangulate.py

@@ -127,6 +127,8 @@ def test_delaunay_triples_invalid_format(dataframe):
     """
     with pytest.raises(GMTInvalidInput):
         triangulate.delaunay_triples(data=dataframe, output_type=1)
+    with pytest.raises(GMTInvalidInput):
+        triangulate.delaunay_triples(data=dataframe, output_type="pandas", header="o+c")
 
 
 def test_regular_grid_with_outgrid_true(dataframe, expected_grid):