Add ILAMB into diagnostics workflow

[rosiealice]

Presently the land diagnostics package is entirely 'diagnostic' in that there are either no data (for Marit's package) or very few data (for the NCL package). To ge to the point where we can assess the skill of the land model, we would need to either

1. bring additional data products into the diagnostics package, or
2. run ILAMB, which is a dedicated package for comparing and testing against a huge and updated raft of land surface datasets.

https://ilamb.org/software.html

While we might end up doing 1) to some extent I think it is important to plan on implementing ILAMB within the main workflow if possible.