/// This performs a simultaneous write/read transaction over the selected SPI bus. Data that was in your buffer is overwritten by data returned from the SPI bus.
/// That’s all there is in the helper library. It is possible to do simple read and writes over the SPI bus using the standard read() and write() system calls though –
/// write() may be better to use for sending data to chains of shift registers, or those LED strings where you send RGB triplets of data.
/// Devices such as A/D and D/A converters usually need to perform a concurrent write/read transaction to work.
/// This is the way to initialize a channel (The Pi has 2 channels; 0 and 1). The speed parameter is an integer
/// in the range 500,000 through 32,000,000 and represents the SPI clock speed in Hz.
/// The returned value is the Linux file-descriptor for the device, or -1 on error. If an error has happened, you may use the standard errno global variable to see why.
/// </summary>
/// <param name="channel">The channel.</param>
/// <param name="speed">The speed.</param>
/// <returns>The Linux file descriptor for the device or -1 for error.</returns>