allow multiple axes in Array.diff #1135
Description
Our users models often have year and age axes and when using diff on such an array (this is rare though), it should be on both axes at the same time. They currently have to resort to ugly code to do so because diff only supports a single axis.
FWIW, numpy also supports only a single axis for diff (but our implementation of diff does not use numpy's) but their gradient implementation supports multiple axes
low-priority because it is rarely needed. I still think we should implement this some day because it just generalize an existing api.