ZH: show phase due to t_trace[0] from measurement

This is not required to be known when finding the phase
as lib.find_beacon_in_traces already accounts for it.

simulations/airshower_beacon_simulation/lib/tests/test_beacon_fourier.py

@@ -1,5 +1,12 @@
 #!/usr/bin/env python3
 
+"""
+Test the functions in lib concerning
+beacon generation and phase measuring
+work correctly together.
+"""
+
+
 import matplotlib.pyplot as plt
 import numpy as np
 
@@ -7,20 +14,25 @@ import lib
 
 seed = 12345
 dt = 1 # ns
-t = np.arange(0, 10*int(1e3), dt)
 frequency = 45e-3 # GHz
-
 N = 5e2
+
+t = np.arange(0, 10*int(1e3), dt, dtype=float)
 rng = np.random.default_rng(seed)
 
 phase_res = np.zeros(int(N))
 
+# Vary both the base time and the phase
 for i in range(int(N)):
+    t_extra = (2*rng.uniform(size=1) - 1) *1e3
+    t += t_extra
+
     phase = lib.phase_mod(np.pi*(2*rng.uniform(size=1) -1)) # rad
     beacon = lib.sine_beacon(frequency, t, t0=0, phase=phase)
 
     measured = lib.find_beacon_in_traces([beacon], t, frequency, frequency_fit=False)
 
+    t -= t_extra
     phase_res[i] = lib.phase_mod(measured[1][0] - phase)
 
 fig, ax = plt.subplots()