Remove TODOs in model

[weswigham]

This tracks a collection of in-code TODOs around here. Given the vega-lite API,
hours, for example, could be typed as 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23, rather than number and be much more accurate, in terms of what is allowable. unfortunately, TS doesn't (yet) guard on less than/greater than, so this could be super cumbersome for API consumers (if they have a function which takes a number, after they've verified its range themselves, they'd need to cast to this union or a member of it). Obviously, then milliseconds is even longer.

Y'know, the more I write the more I realize with current limitations, this is a bad idea. This issue now tracks removing those TODOs from the code - until TS supports numeric range types and guards for them, specifying allowed numbers like this is just too cumbersome for API definers and consumers.