Please add greyworld to auto white balance (AWB_MODES = 'greyworld')

[ReyLoBacca]

For the NoIR camera module, greyworld is a valid and very useful awb mode. Could it be as simple as this to add it?

You can read about greyworld here:

https://www.raspberrypi.org/documentation/raspbian/applications/camera.md

AWB_MODES = {
        'off':           mmal.MMAL_PARAM_AWBMODE_OFF,
        'auto':          mmal.MMAL_PARAM_AWBMODE_AUTO,
        'sunlight':      mmal.MMAL_PARAM_AWBMODE_SUNLIGHT,
        'cloudy':        mmal.MMAL_PARAM_AWBMODE_CLOUDY,
        'shade':         mmal.MMAL_PARAM_AWBMODE_SHADE,
        'tungsten':      mmal.MMAL_PARAM_AWBMODE_TUNGSTEN,
        'fluorescent':   mmal.MMAL_PARAM_AWBMODE_FLUORESCENT,
        'incandescent':  mmal.MMAL_PARAM_AWBMODE_INCANDESCENT,
        'flash':         mmal.MMAL_PARAM_AWBMODE_FLASH,
        'horizon':       mmal.MMAL_PARAM_AWBMODE_HORIZON,
       'greyworld':   mmal.MMAL_PARAM_AWBMODE_GREYWORLD
        }