Arduino driver for PMW3901 optical flow sensor

This library is based on on the C driver code for the Crazyflie quadcopter from Bitcraze. Unlike the Bitcraze PMW3901 Arduino repository, their C driver provides a data structure that includes a flag for whether motion was detected, which may be useful to know for sensor fusion. Hence my C++ class returns this flag along with the delta-X and delta-Y motion values. The class offers the following features:

1. Uses header-only C++

2. Supports different SPI buses (e.g., Arduino SPI1)