Add property scale_factor0 to FLRW cosmologies

This PR is part of a sequence to implement the Cosmology API.

Signed-off-by: Nathaniel Starkman <nstarman@users.noreply.github.com>

astropy/cosmology/flrw/base.py

@@ -66,7 +66,32 @@ def quad(*args, **kwargs):
 ##############################################################################
 
 
-class FLRW(Cosmology):
+class _ScaleFactorMixin:
+    @property
+    def scale_factor0(self):
+        """Scale factor at redshift 0."""
+        return 1.0
+
+    def scale_factor(self, z):
+        """Scale factor at redshift ``z``.
+
+        The scale factor is defined as :math:`a = 1 / (1 + z)`.
+
+        Parameters
+        ----------
+        z : Quantity-like ['redshift'], array-like, or `~numbers.Number`
+            Input redshift.
+
+        Returns
+        -------
+        a : ndarray or float
+            Scale factor at each input redshift.
+            Returns `float` if the input is scalar.
+        """
+        return 1.0 / (aszarr(z) + 1.0)
+
+
+class FLRW(Cosmology, _ScaleFactorMixin):
     """
     A class describing an isotropic and homogeneous
     (Friedmann-Lemaitre-Robertson-Walker) cosmology.
@@ -900,24 +925,6 @@ def H(self, z):
         """
         return self._H0 * self.efunc(z)
 
-    def scale_factor(self, z):
-        """Scale factor at redshift ``z``.
-
-        The scale factor is defined as :math:`a = 1 / (1 + z)`.
-
-        Parameters
-        ----------
-        z : Quantity-like ['redshift'], array-like, or `~numbers.Number`
-            Input redshift.
-
-        Returns
-        -------
-        a : ndarray or float
-            Scale factor at each input redshift.
-            Returns `float` if the input is scalar.
-        """
-        return 1.0 / (aszarr(z) + 1.0)
-
     def lookback_time(self, z):
         """Lookback time in Gyr to redshift ``z``.