Return error if chunk over an inner dim that crosses a file dim.

Hi @nperez

In this branch, an error message is added if "<inner_dim>_across = <file_dim> is used + merge_across_dims = FALSE + chunk over <inner_dim>". It is a problem found in issue #51 (closed).

Please let me know if you have any questions about it, or it's fine to merge it.

Cheers,
An-Chi