only calling toframe with variables used in a given model

[fscottfoti]

@jiffyclub You know how in UrbanSim we only compute the variables that are actually used in the yaml specs? We will need the same thing for the csv specs. Right now it computes all the variables and in a lot of cases variables simply aren't available until later on in the model chain. No hurry as I can work around it, but wanted to make a note.

[fscottfoti]

I think this is not an issue. This is different than UrbanSim where a lot of variables change after each model. In other words, households and persons don't change composition during the day, so most of the variables can be cached and left. We might compute more than we have at the beginning of the simulations but it won't cost us computation over the whole simulation (as long as we just enable caching).