add some automation to pi side

pi/roverd/sensor_defs.go

@@ -0,0 +1,23 @@
+package roverd
+
+var (
+	defaultStreamPackets = []byte{100, 21, 34}
+	packetSizes          = map[byte]int{
+		100: 80,
+		21:  1,
+		34:  1,
+	}
+	expectedPayloadLength = func() int {
+		sum := 0
+		for _, id := range defaultStreamPackets {
+			sum += 1 + packetSizes[id]
+		}
+		return sum
+	}()
+)
+
+type SensorSample struct {
+	Timestamp     int64
+	ChargingState byte
+	ChargeSources byte
+}