Correct bug in step function

Author: achiefa

validphys2/src/validphys/theorycovariance/higher_twist_functions.py

@@ -70,15 +70,14 @@ def step_function(a: npt.ArrayLike, y_shift: npt.ArrayLike, bin_edges: npt.Array
     A one-dimensional array containing the function values evaluated at the points
     specified in `a`.
     """
-    res = []
+    res = np.zeros_like(a)
     for shift_pos, shift in enumerate(y_shift):
         bin_low = bin_edges[shift_pos]
         bin_high = bin_edges[shift_pos + 1]
         condition = np.multiply(
             a >= bin_low, a < bin_high if shift_pos != len(y_shift) - 1 else a <= bin_high
         )
-        res.append([shift for cond in condition if cond])
-    res = np.concatenate(res)
+        res = np.add(res, [shift if cond else 0.0 for cond in condition])
     return res