Scale value error

Fixes #59 (closed)

geomagio/residual/Calculation.py

-from typing import List, Tuple
+from typing import List, Optional, Tuple
 from typing_extensions import Literal
 
 import numpy as np
@@ -255,7 +255,7 @@ def calculate_I(
 
 
 def calculate_scale_value(
-    measurements: List[Measurement], inclination: float, corrected_f: float
+    measurements: List[Measurement], inclination: float, corrected_f: Optional[float]
 ) -> float:
     """Calculate scale value.
 
@@ -269,6 +269,8 @@ def calculate_scale_value(
     Returns
     -------
     Calculated scale value.
+
+    Note: scale value is None if residual values between NorthDownScale measurements are the same
     """
     inclination_radians = np.radians(inclination)
     m1, m2 = measurements[0], measurements[-1]
@@ -280,5 +282,7 @@ def calculate_scale_value(
         / corrected_f
     ) + (m2.angle - m1.angle)
     residual_change = m2.residual - m1.residual
+    if residual_change == 0:
+        return None
     scale_value = corrected_f * field_change / np.abs(residual_change)
     return scale_value