Adding test of Hubble parameter

tests/test_background.py

@@ -6,6 +6,37 @@
 import jax_cosmo.background as bkgrd
 from jax_cosmo import Cosmology
 
+def test_H():
+     # We first define equivalent CCL and jax_cosmo cosmologies
+    cosmo_ccl = ccl.Cosmology(
+        Omega_c=0.3,
+        Omega_b=0.05,
+        h=0.7,
+        sigma8=0.8,
+        n_s=0.96,
+        Neff=0,
+        transfer_function="eisenstein_hu",
+        matter_power_spectrum="linear",
+        wa=2. # non-zero wa 
+    )
+
+    cosmo_jax = Cosmology(
+        Omega_c=0.3,
+        Omega_b=0.05,
+        h=0.7,
+        sigma8=0.8,
+        n_s=0.96,
+        Omega_k=0.0,
+        w0=-1.0,
+        wa=2.0, # non-zero wa 
+    )
+
+    # Test array of scale factors
+    a = np.linspace(0.01, 1.0)
+
+    H_ccl = ccl.h_over_h0(cosmo_ccl, a)
+    H_jax = bkgrd.H(cosmo_jax, a) / 100.
+    assert_allclose(H_ccl, H_jax, rtol=1.e-3)
 
 def test_distances_flat():
     # We first define equivalent CCL and jax_cosmo cosmologies