Add a `split` parameter to hconcat and vconcat functions to specify fractions

[keller-mark]

Currently, hconcat and vconcat split the views equally. If two views are provided, they split into halfs. If three are provided, then thirds, etc.

We could add a new parameter to allow the exact breakdown to be specified by the user such as

hconcat(v1, v2, split=[1,2])

which would make v1 1/3 width and v2 2/3 width.

The denominator would just be the sum of the values