Fix Choice down spacing

[robjhornby]

Nice library!

I noticed a problem in the Python package and have a fix for it (PR to follow). Here's a diagram which shows the problem:

from railroad import Choice, Diagram, Group, Skip, Stack, Terminal

diagram = Diagram(
    Choice(
        0,
        Group(Choice(0, Skip(), Group(Stack(Terminal("abc"), Skip()), "inner")), "top"),
        Terminal("xyz"),
    )
)

With the current latest commit in this repo, it looks like:

I think the down spacing of Choice is where the problem lies. After fixing it it looks like this:

I'm not sure how to run the tests in this repo so I just got this working with the above example and haven't checked whether it breaks anything else.

[tabatkins]

Hm, looking at the code now, I'm pretty sure it's wronger than what #107 is fixing. I'll take a deeper look at this tomorrow.

[tabatkins]

And yup, the code was still making a lot of assumptions that predated the introduction of the "height" concept. All fixed now!

[robjhornby]

Thanks for the quick fix, looks good!