fix Signalpath.phase(): do not modify original data in place

phase() overwrote the underlying channel arrays (y[i] *= -1) and returned
self, so rotated copies aliased the original object and negative phase
angles garbled shared data. It now returns a copy with new arrays and
uses numpy.deg2rad for arbitrary angles.
Add unittests for phase() including negative angles and non-mutation.

src/damaris/data/Signalpath.py

@@ -1,3 +1,5 @@
+import copy
+
 import numpy as N
 
 class Signalpath:
@@ -5,33 +7,35 @@ class Signalpath:
         """
         rotate signal by **degrees** for phase cycling, etc.
 
+        The signalpath object itself is not modified, a copy containing the
+        rotated data is returned.
+
         :param degrees: rotate signal by this value
-        :return:
+        :return: copy of the signalpath object with rotated data
         """
         if self.get_number_of_channels() != 2:
             raise Exception("rotation defined only for 2 channels")
+        rotated = copy.copy(self)
         # simple case 0, 90, 180, 270 degree
         reduced_angle = divmod(degrees, 90)
         if abs(reduced_angle[1]) < 1e-6:
             reduced_angle = reduced_angle[0] % 4
             if reduced_angle == 0:
-                return self
+                rotated.y = list(self.y)
 
             elif reduced_angle == 1:
-                self.y[1] *= -1
-                self.y = [self.y[1], self.y[0]]
+                rotated.y = [-self.y[1], self.y[0]]
 
             elif reduced_angle == 2:
-                self.y[0] *= -1
-                self.y[1] *= -1
+                rotated.y = [-self.y[0], -self.y[1]]
 
             elif reduced_angle == 3:
-                self.y[0] *= -1
-                self.y = [self.y[1], self.y[0]]
+                rotated.y = [self.y[1], -self.y[0]]
         else:
-            sin_angle = N.sin(degrees / 180.0 * N.pi)
-            cos_angle = N.cos(degrees / 180.0 * N.pi)
-            self.y = [cos_angle * self.y[0] - sin_angle * self.y[1],
-                      sin_angle * self.y[0] + cos_angle * self.y[1]]
+            rad_angle = N.deg2rad(degrees)
+            sin_angle = N.sin(rad_angle)
+            cos_angle = N.cos(rad_angle)
+            rotated.y = [cos_angle * self.y[0] - sin_angle * self.y[1],
+                         sin_angle * self.y[0] + cos_angle * self.y[1]]
 
-        return self
+        return rotated