Refactor ARTIS density and mass fractions readers for consistency and…

… clarity

tardis/io/model/readers/artis.py

@@ -115,21 +115,21 @@ def read_artis_density(fname, legacy_return=True):
     isotope_mass_fractions.columns.name = "cell_index"
 
     if legacy_return:
-        return time_of_model, velocity, mean_density
+        return time_of_model, velocity, mean_density[1:]
     else:
         return time_of_model, velocity, mean_density, isotope_mass_fractions
 
 
 def read_artis_mass_fractions(fname, normalize=True):
     """
-    Reads mass fractions from an artis abundance file named e.g. 'artis_abundances.dat'.
+    Reads mass fractions from an ARTIS abundance file named e.g. 'artis_abundances.dat'.
     Each row typically corresponds to one shell. The first column is shell index,
     followed by columns for each element.
 
     Parameters
     ----------
     fname : str
-        filename or path to artis abundances
+        filename or path to ARTIS abundances
 
     Returns
     -------

tardis/io/model/readers/tests/test_artis_readers.py

@@ -1,5 +1,6 @@
 import numpy as np
 import numpy.testing as npt
+import pytest
 from astropy import units as u
 
 from tardis.io.model.readers.artis import (
@@ -62,7 +63,7 @@ def test_artis_model_reader(example_model_file_dir):
 
 
 def test_simple_legacy_read_artis_density(artis_density_fname: str):
-    time_of_model, velocity, mean_density, isotope_mass_fractions = read_artis_density(
+    time_of_model, velocity, mean_density = read_artis_density(
         artis_density_fname, legacy_return=True
     )