MVP Correction Implemented

Add implementation for the basic set of spectrum components:

Primary
-------
- Phase Space
- Fermi Function

Corrections
-----------

- Atomic:
  - Screening
  - Atomic exchange
- E/M and finite size:
  - Finite nucleus size and charge distribution
  - Radiative

beta_spectrum/utils.py

@@ -1,9 +1,27 @@
 # utils.py
+import numpy as np
+
 from .constants import HBAR_C_MEV_FM, ME_MEV
 
 
-def nuclear_radius(A: float) -> float:
+def nuclear_radius(A: float, r0: float = 1.2) -> float:
     r0_fm = 1.2  # fm
     R_fm = r0_fm * A ** (1 / 3)
 
     return R_fm / (HBAR_C_MEV_FM / ME_MEV)
+
+
+def momentum(W):
+    return np.sqrt(W**2 - 1)
+
+
+def neutrino_energy(W, W0):
+    return W0 - W
+
+
+def W_to_T(W: float | np.ndarray) -> np.ndarray:
+    return (W - 1.0) * ME_MEV
+
+
+def T_to_W(T_keV: float | np.ndarray) -> np.ndarray:
+    return T_keV / ME_MEV + 1.0