Allow fi_pool() to take a list of poolsizes and numbers (to match fi_pool_cluster)

[AngusMcLure]

This would be good for consistency across the fi_ family of functions. Also, it would allow the implementation of a fi_pool_random() much more easily and consistently with fi_pool_cluster_random().

Downside is that it is slightly more clunky to vectorise fi_pool wrt pool_size and pool_number, but I don't think is major. Would need to check that fi_pool isn't being used in vectorised form already.