scd4x: SCD4x CO2 Sensor - Initial Add (#81)

* Initial Add

* Fix lint issue and inversion of skip testing

scd4x/README.md

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+# Sensirion SCD4x CO<sub>2</sub> Sensors
+
+## Overview
+
+This package provides a driver for the Sensirion SCD4x CO<sub>2</sub> sensors. This is a 
+compact sensor that provides temperature, humidity, and CO<sub>2</sub> concentration 
+readings. The datasheet for this device is available at:
+
+https://sensirion.com/media/documents/48C4B7FB/66E05452/CD_DS_SCD4x_Datasheet_D1.pdf
+
+## Testing
+
+The unit tests can function with either a live sensor, or in playback mode. If the 
+environment variable SCD4X is set, then the self test code will use a live 
+sensor on the default I<sup>2</sup>C bus. For example:
+
+```bash
+$> SCD4X=1 go test -v
+```
+If the environment variable is not present, then unit tests will be conducted using
+playback values.
+
+## Notes
+
+### Acquisition Time
+
+The minimum acquisition time for the sensor is 5 seconds. If you call Sense() more
+frequently, it will block until a reading is ready.
+
+### Forced Calibration and Self-Test
+
+These functions are not implemented. From examining the datasheet, and 
+experimenting, it appears that these two calls require the i2c communication 
+driver to wait a specified period before initiating the read. The periph.io 
+I<sup>2</sup>C library doesn't support this functionality. This means that attempts 
+to call these functions will always fail so they're not implemented.
+
+### Acquisition Mode
+
+Only certain commands can be issued while the device is running in acquisition 
+mode. If you're working on the low-level code, be aware that attempts to send
+a non-allowed command while in acquisition mode will return an i2c remote 
+io-error.
+
+### Automatic Self Calibration
+
+When Automatic Self Calibration is enabled, and the sensor has run for the 
+required period, it will adjust itself so that the LOWEST recorded reading 
+during the period yields the value set for ASC Target. The factory default 
+target is 400PPM, but the current PPM is ~425PPM. To get a more accurate 
+value for CO2 concentration in Earth's atmosphere, refer to:
+
+https://www.co2.earth/daily-co2
+
+For more details, refer to the datasheet.
+

scd4x/doc.go

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+// Copyright 2024 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+// This package provides a driver for the Sensiron SCD4x CO2 sensors.
+// The scd4x family provide a compact sensor that can be used to measure
+// Temperature, Humidity, and CO2 concentration.
+//
+// Refer to the datasheet for more information.
+//
+// https://sensirion.com/media/documents/48C4B7FB/66E05452/CD_DS_SCD4x_Datasheet_D1.pdf
+package scd4x

scd4x/example_test.go

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+//go:build examples
+// +build examples
+
+// Copyright 2024 The Periph Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package scd4x_test
+
+import (
+	"fmt"
+	"log"
+
+	"periph.io/x/conn/v3/i2c/i2creg"
+	"periph.io/x/devices/v3/scd4x"
+	"periph.io/x/host/v3"
+)
+
+// basic example program for scd4x sensors using this library.
+//
+// To execute this as a stand-alone program:
+//
+// Copy the file example_test.go to a new directory.
+// rename the file to main.go
+// rename the Example() function to main, and the package to main
+//
+// execute:
+//
+//	go mod init mydomain.com/scd4x
+//	go mod tidy
+//	go build -o main main.go
+//	./main
+func Example() {
+	fmt.Println("scd4x example program")
+	if _, err := host.Init(); err != nil {
+		fmt.Println(err)
+	}
+	bus, err := i2creg.Open("")
+	if err != nil {
+		log.Fatal(err)
+	}
+	dev, err := scd4x.NewI2C(bus, scd4x.SensorAddress)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	env := scd4x.Env{}
+	err = dev.Sense(&env)
+	if err == nil {
+		fmt.Println(env.String())
+	} else {
+		fmt.Println(err)
+	}
+
+	cfg, err := dev.GetConfiguration()
+	if err == nil {
+		fmt.Printf("Configuration: %#v\n", cfg)
+	} else {
+		fmt.Println(err)
+	}
+	// Output: Temperature: 24.845°C Humidity: 32.3%rH CO2: 581 PPM
+	// Configuration: &scd4x.DevConfig{AmbientPressure:0, ASCEnabled:true, ASCInitialPeriod:158400000000000, ASCStandardPeriod:561600000000000, ASCTarget:400, SensorAltitude:0, SerialNumber:127207989525260, TemperatureOffset:4, SensorType:0}
+}