Remove Timestep code path for ParmedConverter (#3172)

Towards #3031 

# Work done in this PR
* Removes the non-working code path for timestep handling in the ParmedConverter

package/CHANGELOG

@@ -31,6 +31,8 @@ Changes
   * Maximum pinned versions in setup.py removed for python 3.6+ (PR #3139)
 
 Deprecations
+  * ParmEdConverter no longer accepts Timestep objects at all
+    (Issue #3031, PR #3172)
   * NCDFWriter `scale_factor` writing will change in version 2.0 to
     better match AMBER outputs (Issue #2327)
   * Deprecated using the last letter of the segid as the

package/MDAnalysis/coordinates/ParmEd.py

@@ -76,6 +76,7 @@
 import functools
 import itertools
 import warnings
+from six import raise_from
 
 from . import base
 from ..topology.tables import SYMB2Z
@@ -174,9 +175,10 @@ def convert(self, obj):
             ag_or_ts = obj.atoms
         except AttributeError:
             if isinstance(obj, base.Timestep):
-                ag_or_ts = obj.copy()
+                raise ValueError("Writing Timesteps to ParmEd "
+                                 "objects is not supported")
             else:
-                raise_from(TypeError("No Timestep found in obj argument"), None)
+                raise_from(TypeError("No atoms found in obj argument"), None)
 
         # Check for topology information
         missing_topology = []

testsuite/MDAnalysisTests/coordinates/test_parmed.py

@@ -31,6 +31,7 @@
 
 from MDAnalysisTests.coordinates.base import _SingleFrameReader
 from MDAnalysisTests.coordinates.reference import RefAdKSmall
+from MDAnalysis.coordinates.ParmEd import ParmEdConverter
 
 from MDAnalysisTests.datafiles import (
     GRO,
@@ -288,3 +289,17 @@ class TestParmEdConverterPDB(BaseTestParmEdConverter):
     def test_equivalent_coordinates(self, ref, output):
         assert_almost_equal(ref.coordinates, output.coordinates, decimal=3)
 
+
+def test_pass_ts_error():
+    u = mda.Universe(PDB_small)
+    err = "Writing Timesteps to ParmEd objects is not supported"
+    with pytest.raises(ValueError, match=err):
+        c = ParmEdConverter()
+        c.convert(u.trajectory.ts)
+
+
+def test_incorrect_object_passed_typeerror():
+    err = "No atoms found in obj argument"
+    with pytest.raises(TypeError, match=err):
+        c = ParmEdConverter()
+        c.convert("we still don't support emojis :(")