working on speeding up workplace assignment
Adding distance_distributions.cpp

This is the code that constructs the immunity swapping file.  Not sure why it
was not previously checked in.
Adding script that assigns people to workplaces and schools

School assignment is based on simple proximity, i.e. students are
sent to the nearest school.

Workplace assignment is probabilistic according to a gravity model
that takes into account the distance to between home and the workplace,
and the number of jobs available at that workplace.  The 1000 nearest
workplaces are sampled from.
Updating process_csv_extracts.py

These are updates to the script that process the csv extracts of the Excel files
from Hector.  I'm not sure why they weren't committed before, but they look
legit.
Trivial changes, changed expansion factor to 12