official_models/pup/trains_sensors: tilt project

This adds a project that shows how you can combine LEGO trains with the
Powered Up Tilt Sensor to detect slopes in a train track.

official_models/pup/trains_sensors/slope_detection/README.md

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+# Rail slope detection
+
+Made by: The Pybricks Authors
+
+## Introduction
+
+This project shows how you can integrate the
+[Tilt Sensor](https://docs.pybricks.com/en/latest/pupdevices.html#tilt-sensor)
+in your LEGO trains. You can use this sensor to detect a slope in the rail
+track.
+
+## Demo
+
+This program makes the train drive until it detects a slope of 8 degrees or
+more. Then it drives back down. You can watch a video
+[here](https://www.youtube.com/watch?v=Wc0mRIBjhIU).
+
+## Requirements
+- Generic LEGO train, such as 60197 or 60198
+- Powered Up [Tilt Sensor](https://docs.pybricks.com/en/latest/pupdevices.html#tilt-sensor)
+
+## Building instructions
+Build the train using the standard instructions. Mount the Tilt Sensor anywhere
+on the train. See the video for an example. Mount it horizontally. The side
+with the cable should face towards the front: in the direction of driving.
+
+## Ports
+- Train motor on port A 
+- Tilt sensor on port B
+
+## Programming and running instructions
+The train must start on a horizontal track. Then run the program.

official_models/pup/trains_sensors/slope_detection/slope.py

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+from pybricks.pupdevices import TiltSensor, DCMotor
+from pybricks.parameters import Port
+from pybricks.tools import wait
+
+# Initialize the train motor.
+# If you have a motor with encoders, use the Motor class instead.
+train = DCMotor(Port.A)
+
+# Initialize the tilt sensor.
+sensor = TiltSensor(Port.B)
+
+# Measure the tilt while the train is on a flat surface.
+pitch_start, roll_start = sensor.tilt()
+
+# Start driving.
+train.dc(-50)
+
+# We need to reach a constant speed before we can check tilt again,
+# because acceleration affects tilt. So we wait a second.
+wait(1000)
+
+# Wait for the train to sense the hill.
+while True:
+    pitch_now, roll_now = sensor.tilt()
+
+    wait(10)
+
+    # If we reached an extra 3 degrees, exit/break the loop.
+    if pitch_now >= pitch_start + 8:
+        break
+
+# Drive backwards for a few seconds and then stop.
+train.dc(40)
+wait(3000)
+train.dc(0)