ENH: Allow chained splitting of different fields

[tclose]

What would you like changed/added and why?

Would like to support chained splitting, i.e. for

node.split(a=[1, 2]).split(b=[3, 4])

to be equivalent to

node.split(a=[1, 2], b=[3, 4])

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

More flexible syntax made available to the end user

[effigies]

This would allow you to do something like:

node.split(a=[1, 2])
if cond:
    node.split(b=[3, 4])

instead of

if cond:
    node.split(a=[1, 2])
else:
    node.split(a=[1, 2], b=[3, 4])

or

splits = {'a': [1, 2]}
if cond:
    splits['b'] = [3, 4]
node.split(**splits)

As long as splits are "outer" by default, there is no ambiguity in adding splits sequentially. A downstream node is already split by its upstream nodes.