refactor code into modules

src/pg_rad/isotopes/isotope.py

@@ -0,0 +1,23 @@
+class Isotope:
+    """Represents the essential information of an isotope.
+
+    Args:
+        name (str): Full name (e.g. Caesium-137).
+        E (float): Energy of the primary gamma in keV.
+        b (float): Branching ratio for the gamma at energy E.
+    """  
+    def __init__(
+            self,
+            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)")
+
+        self.name = name
+        self.E = E
+        self.b = b