Change the default primoridal Helium fraction to the value from WMAP7 that was...

Change the default primoridal Helium fraction to the value from WMAP7 that was assumed in all the EAGLE and OWLS runs.

src/physical_constants_cgs.h

@@ -36,6 +36,8 @@
  * Phys. C, 40, 100001 (2016) and 2017 update.
  * http://pdg.lbl.gov/2017/reviews/rpp2017-rev-phys-constants.pdf
  * http://pdg.lbl.gov/2017/reviews/rpp2017-rev-astrophysical-constants.pdf
+ *
+ * The primordial Helium fraction is the value obtained by WMAP7.
  */
 
 /*! Newton's gravitation constant [g^-1 cm^3 s^-2] */
@@ -95,8 +97,8 @@ const double const_earth_mass_cgs = 5.9724e27;
 /*! Temperature of the CMB at present day [K] */
 const double const_T_CMB_0_cgs = 2.7255;
 
-/*! Primordial Helium fraction [-] */
-const double const_primordial_He_fraction_cgs = 0.245;
+/*! Primordial Helium fraction (from WMAP7) [-] */
+const double const_primordial_He_fraction_cgs = 0.248;
 
 /*! Reduced Hubble constant units (i.e. H_0 / h == 100 km / s / Mpc in CGS)
  * [s^-1] */