to_df() backwards compatibility

Specifically for `to_df(np.s[...])` calls that
did not name an argument

Author: ax3l

src/binding/python/openpmd_api/DataFrame.py

@@ -10,7 +10,10 @@
 import numpy as np
 
 
-def particles_to_dataframe(particle_species, *legacy_args, attributes=None, slice=None):
+def particles_to_dataframe(particle_species,
+                           *legacy_args,
+                           attributes=None,
+                           slice=None):
     """
     Load all records of a particle species into a Pandas DataFrame.
 
@@ -52,10 +55,10 @@ def particles_to_dataframe(particle_species, *legacy_args, attributes=None, slic
             slice = legacy_args[0]
             import warnings
             warnings.warn("The to_df() argument order changed in "
-                "openPMD-api 0.17.0!\n"
-                "The slice= argument must be passed as a named argument.",
-                DeprecationWarning
-            )
+                          "openPMD-api 0.17.0!\n"
+                          "The slice= argument must be passed as a named argument.",
+                          DeprecationWarning
+                          )
         else:
             raise RuntimeError("to_df() does not support unnamed arguments!")