Change Horizon constraint to allow loading from lists

[wtgee]

We currently restrict it to a list of tuples of tuples but that makes writing it in yaml less intuitive. Should be fine to accept a list of lists of list. :)

i.e. now only:

[
        ((40, 45), (50, 50), (60, 45))
]

but should accept:

[
        [[40, 45], [50, 50], [60, 45]]
]

[jermainegug]

We can now use a list of lists for our obstructions. :)

[wtgee]

We can now use a list of lists for our obstructions. :)

Does this mean a PR is coming? :) I don't think I changed it yet.