Replace container_ndim with split_dims arg

[tclose]

What would you like changed/added and why?

The container_ndim (formerly cont_dim) be replaced by split_dims. Where as container_ndim only allows you to specify the number of outer dimensions to be unravelled and split over, e.g. container_ndim = {"Mul.a": 2}, split_dims could let you select arbitrary subsets of dimensions to be split over, e.g. split_dims = {"Mul.a": [0, 2, 3])

What would be the benefit? Does the change make something easier to use?

Enables arbitrary unravelling of multi-dimensional arrays and list-of-lists-of-lists...

Clarifies something?

I think split_dims is a little more intuitive than container_ndim in terms of what it is specifying