fix #43 Signalpath does not return object when phase=0

src/damaris/data/Signalpath.py

@@ -15,7 +15,7 @@ class Signalpath:
         if abs(reduced_angle[1]) < 1e-6:
             reduced_angle = reduced_angle[0] % 4
             if reduced_angle == 0:
-                return
+                return self
 
             elif reduced_angle == 1:
                 self.y[1] *= -1