adds new function to calculate the grav radius

src/slang/physics/gravitational_radius.sl

+%!%+
+%\function{gravitational_radius}
+%\synopsis{calculates the gravitational radius in Meters defined as r_g=GM/c^,
+% for a mass given in units of M_sol.}
+%\usage{gravitational_radius(mass_in_solar)}
+%!%-
+define gravitational_radius() {
+   variable mass_in_solar;
+   
+   switch(_NARGS)
+   { case 1: mass_in_solar = ();}
+   { help(_function_name()); return; }
+
+   variable G = 6.673e-11;
+   variable c = 299792458.0;
+   variable Msol = 1.988e30;  % in kg
+   
+   return    G*Msol*mass_in_solar / (c^2);
+}