average an image along a specific axis.
| -axis axis | specify axis along which averaging should be performed. By default, the program will use the last non-singleton axis of the input image. 
| axis | the concatenation axis |  | 
| -geometric | produce geometric mean. By default, the program will produce the arithmetic mean. | 
| -info | display information messages. | 
| -quiet | do not display information messages or progress status. | 
| -debug | display debugging messages. | 
| -help | display this information page and exit. | 
| -version | display version information and exit. |