Initial commit

Base modular structure (phase space, Fermi function)
Initial .gitignore

beta_spectrum/units.py

@@ -0,0 +1,12 @@
+# units.py
+import numpy as np
+
+ME_KEV = 510.99895000  # keV electron mass
+
+
+def W_to_T(W: float | np.ndarray) -> np.ndarray:
+    return (W - 1.0) * ME_KEV
+
+
+def T_to_W(T_keV: float | np.ndarray) -> np.ndarray:
+    return T_keV / ME_KEV + 1.0