snippets/pup/sensor_ultrasonic: add math example

This demonstrates functionality of the lights and builtin math.

snippets/pup/sensor_ultrasonic/math.py

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+from pybricks.pupdevices import UltrasonicSensor
+from pybricks.parameters import Port
+from pybricks.tools import wait, StopWatch
+
+# The math module is part of standard MicroPython:
+# https://docs.micropython.org/en/latest/library/math.html
+from math import pi, sin
+
+# Initialize the sensor.
+eyes = UltrasonicSensor(Port.A)
+
+# Initialize a timer.
+watch = StopWatch()
+
+# We want one full light cycle to last three seconds.
+PERIOD = 3000
+
+while True:
+    # The phase is where we are in the unit circle now.
+    phase = watch.time()/PERIOD*2*pi
+
+    # Each light follows a sine wave with a mean of 50, with an amplitude of 50.
+    # We offset this sine wave by 90 degrees for each light, so that all the
+    # lights do something different.
+    brightness = [sin(phase + offset*pi/2) * 50 + 50 for offset in range(4)]
+
+    # Set the brightness values for all lights. The * symbol unpacks the list
+    # of brightness values into separate arguments.
+    eyes.lights.on(*brightness)
+
+    # Wait some time.
+    wait(50)