Add support for external data variables with > 2 dimensions

[chrispcampbell]

We expanded support for external data variables in #32 but it only handled up to 2 dimensions. The En-ROADS model includes some 3-dimensional external data variables, so we should update the code to handle this case.

Additionally, there was a TODO comment about simplifying the code, and I think we can address that here by generalizing the dimension handling code to work with any number of dimensions (without the need for nested loops).