Decorate two_fluid dispersion solver with @particle_input (#2022)

* Decorate two_fluid with @particle_input

* Add initial changelog entry

* Add changelog entry

* Drop usage of Z in function

The charge number is now contained in `ion`.

* Make minor edits to two_fluid

* Update changelog entries

* Allow some parameters to be positional

The ones from that are positional are all distinguishable by
unit or particle-like-ness, so having allowing it to be
positional is not likely to cause any problems.

* Update docstrings

* Unicode!

* Update changelog entries

* Update docstrings and rename variables

* Remove test

changelog/2022.breaking.rst

@@ -0,0 +1,4 @@
+In `~plasmapy.dispersion.analytical.two_fluid_.two_fluid` in
+`plasmapy.dispersion`, providing the |charge number| as a keyword
+argument (formerly ``z_mean``) will no longer override the charge number
+provided in ``ion``.

changelog/2022.feature.rst

@@ -0,0 +1,3 @@
+The function `~plasmapy.dispersion.analytical.two_fluid_.two_fluid`
+has been decorated with |particle_input| and now may accept an
+argument for the mass number corresponding to the ion.

changelog/2022.removal.rst

@@ -0,0 +1,3 @@
+In `plasmapy.dispersion`, the ``z_mean`` parameter
+to `~plasmapy.dispersion.analytical.two_fluid_.two_fluid` has been
+deprecated. Provide the |charge number| to ``Z`` instead.

plasmapy/dispersion/analytical/tests/test_two_fluid_.py

@@ -9,6 +9,7 @@
 from plasmapy.formulary.frequencies import wc_
 from plasmapy.formulary.speeds import cs_, va_
 from plasmapy.particles import Particle
+from plasmapy.particles.exceptions import InvalidIonError
 from plasmapy.utils.exceptions import PhysicsWarning
 
 
@@ -39,8 +40,8 @@ class TestTwoFluid:
             ({**_kwargs_single_valued, "B": -1 * u.T}, ValueError),
             ({**_kwargs_single_valued, "B": 5 * u.m}, u.UnitTypeError),
             ({**_kwargs_single_valued, "ion": {"not": "a particle"}}, TypeError),
-            ({**_kwargs_single_valued, "ion": "e-"}, ValueError),
-            ({**_kwargs_single_valued, "ion": "He", "z_mean": "wrong type"}, TypeError),
+            ({**_kwargs_single_valued, "ion": "e-"}, InvalidIonError),
+            ({**_kwargs_single_valued, "ion": "He", "Z": "wrong type"}, TypeError),
             ({**_kwargs_single_valued, "k": np.ones((3, 2)) * u.rad / u.m}, ValueError),
             ({**_kwargs_single_valued, "k": 0 * u.rad / u.m}, ValueError),
             ({**_kwargs_single_valued, "k": -1.0 * u.rad / u.m}, ValueError),
@@ -71,7 +72,7 @@ def test_raises(self, kwargs, _error):
     @pytest.mark.parametrize(
         ("kwargs", "_warning"),
         [
-            # violates the low-frequency assumption (w/kc << 1)
+            # violates the low-frequency assumption (ω/kc << 1)
             (
                 {
                     "B": 8.3e-7 * u.T,
@@ -139,34 +140,6 @@ def test_on_bellan2012_vals(self, kwargs, expected):
             norm = (np.absolute(val) / (kwargs["k"] * va)).value ** 2
             assert np.isclose(norm, expected[mode])
 
-    @pytest.mark.parametrize(
-        ("kwargs", "expected"),
-        [
-            (
-                {
-                    **_kwargs_bellan2012,
-                    "ion": Particle("He"),
-                    "z_mean": 2.0,
-                    "theta": 0 * u.deg,
-                },
-                {**_kwargs_bellan2012, "ion": Particle("He +2"), "theta": 0 * u.deg},
-            ),
-            #
-            # z_mean defaults to 1
-            (
-                {**_kwargs_bellan2012, "ion": Particle("He"), "theta": 0 * u.deg},
-                {**_kwargs_bellan2012, "ion": Particle("He+"), "theta": 0 * u.deg},
-            ),
-        ],
-    )
-    def test_z_mean_override(self, kwargs, expected):
-        """Test overriding behavior of kw 'z_mean'."""
-        ws = two_fluid(**kwargs)
-        ws_expected = two_fluid(**expected)
-
-        for mode in ws:
-            assert np.isclose(ws[mode], ws_expected[mode], atol=0, rtol=1.7e-4)
-
     @pytest.mark.parametrize(
         ("kwargs", "expected"),
         [