a defined axis should be usable in defining variables from multiple tables

[taylor13]

We should put it on our "to do" list to allow users to define axes that can be used with multiple tables instead of having to define duplicate axes as in the present code.

[taylor13]

To expand on the above, we shouldn't need to define the lat and lon coordinates for two monthly mean fields, just because they happen to be hosted in two separate CMOR tablew (e.g., in Amon and in Lmon), assuming those two fields are reported on the same grid.