Update Phase Space calculation

Add support for proper calculation of phase space component based on the
transition type (allowed/Nth order forbidden).

beta_spectrum/utils.py

@@ -11,8 +11,18 @@ def nuclear_radius(A: float, r0: float = 1.2) -> float:
     return R_fm / (HBAR_C_MEV_FM / ME_MEV)
 
 
-def momentum(W):
-    return np.sqrt(W**2 - 1)
+def momentum(W, rest_mass: float = 1):
+    """
+    Helper function to get momentum value from energy.
+
+    Args:
+        W np.ndarray: Array of full particle energy values in natural units
+        rest_mass float: rest mass of the particle in taural units (m_e = 1 by default for electron)
+
+    Returns:
+        np.ndarray: array of particle momentum values.
+    """
+    return np.sqrt(W**2 - rest_mass**2)
 
 
 def neutrino_energy(W, W0):