replace locals() update with global dictionary

[iliakur]

I knew from somewhere that we really should not be modifying the locals() dictionary as it's part of Python's scaffolding, but didn't vocally object too much because a) there were more important things on our plate and b) I wasn't 100% sure I was right. I now have conclusive proof to back my opinion up and would like to make sure our code adheres to the standard.
Conceptually, it's an easy fix: we simply use the "y" (or whatever we decide to name it) dictionary with our tax parameter names as keys.

In practice this might take some time since we have to go through the entire script changing all the appropriate variable names to dictionary keys.

In case you're worried about performance issues associated with the additional step of looking up the dictionary name, then the key, I have two things to say:

1. Variable lookup is implemented very fast in Python, as it is a fundamental feature. Even with all the hundreds of lookups we'll be introducing, the overhead will be negligeable compared to other bottlenecks.
2. To further speed things up we can pass the name of the dictionary as an argument to our calculating functions. Since Python variable names are references to an object, we won't be shunting too much memory around. At the same time, having a reference to the dictionary in the local namespace of the function will speed up its lookup even further.

[talumbau]

This idea makes sense to me too. +1.

[MattHJensen]

Dealt with in #70 among others.