Add mu_mass_air to isotope automatically calculated from energy and fix error msg

src/pg_rad/isotopes/isotope.py

@@ -1,3 +1,6 @@
+from pg_rad.physics import get_mass_attenuation_coeff
+
+
 class Isotope:
     """Represents the essential information of an isotope.
 
@@ -11,13 +14,14 @@ class Isotope:
             name: str,
             E: float,
             b: float
-    ):        
+    ):
         if E <= 0:
             raise ValueError("primary_gamma must be a positive energy (keV).")
 
         if not (0 <= b <= 1):
-            raise ValueError("branching_ratio_pg must be a ratio (0 <= b <= 1)")
+            raise ValueError("branching_ratio_pg must be a ratio b in [0,1]")
 
         self.name = name
         self.E = E
         self.b = b
+        self.mu_mass_air = get_mass_attenuation_coeff(E / 1000)