add "dimension" to mem() args

[mdsumner]

we need this to avoid having to matrix-ise the values, see the example here:

https://gist.github.com/mdsumner/a4b41c3a3584bc3c5d517d130b3c9e39?permalink_comment_id=4986910#gistcomment-4986910

the data is in the right order, it doesn't need to be an array, but we need to tell DATAPOINTER what the dimensions are (can't just be flat because it could be lower resolution than the array being georeferenced)