Keep an eye out for slowness from std vector copying

[rsmith54]

In things like the RegionVarsCalculator, we are copying the vectors over into the map after the push_backs.

This is definitely slower than allocating the vectors on the heap, and also slower than assigning the map with an empty vector and push_back'ing into a reference to the vector from the map, but I think it's better to keep it since it's a lot simpler as is. IF we have problems with speed at some point we can reconsider this.