Add Isotope object

src/pg_rad/isotopes.py

@@ -0,0 +1,25 @@
+class Isotope:
+    def __init__(
+            self,
+            name: str,
+            E: float,
+            b: float
+    ):
+        """_Represents the essential information of an isotope._
+
+        Args:
+            name (str): _Full name (e.g. Caesium-137)._
+            symbol (str): _Shorthand symbol (e.g. 137Cs or Cs-137)_
+            E (float): _Energy of the primary gamma in keV._
+            b (float): _Branching ratio for the gamma at energy E._
+        """        
+        
+        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)")
+
+        self.name = name
+        self.E = E
+        self.b = b