Fix README file of test_mixture

tests/test_mixture/README.md

@@ -1,19 +1,10 @@
-# test_helium
+# test_mixture
 
-This test is to check calculation of degradation.
+This test is to check calculation of mixed media.
 
-For simplicity, we use the parameters of Helium gas and consider
-4 generations of electrons. (Further generations are obtained using
-the same algorithm.)
+Following Okazaki et al, Bull. Chem. Soc. JPN 49, 1230 (1976), this test
+calculate G values for a mixture of He and Ne gases.
 
-The numerically obtained data are compared with those from 
-WolfranEngine (WE). The WE data are also numerical using NIntegrate.
-Since it is a heavy operation to calculate NIntegrate of NIntegrate-d
-functions, we use Interpolation for the degradation of the previous
-generation (e.g., for y3, we use interpolated y2 obained from NIntegrate
-of y1).
-
-To compare to data, we also use interpolation just in case if the dimensions
-of two data are different. Only data in the valid energy range are compared,
-otherwise the interpolation fails.
-(This treatment leads to unnecessary complication and can be avoided.)
+The reference result is calculated using the same code with commit id
+e67379a.
+