Refactor the instantiation from data

It's a right old mess, the way a Country object is created from data.
Now that I know what the data actually looks like, I'm going to do a
better job. Also, only "The Four Things" really matter.
Reworked services trade balancing
More reworking of services trade balancing

This doesn't work, quite yet. The output to long format is still broken.
Finished reworking services trade balancing

This now actually works. I've run it for 2010 and the longest
convergence was "Public Services" with 473 iterations. The typical
number of iterations is 20.
Changed RoW to match paper

The way the rest of the world (RoW) is handled has changed. This update
reflects that.
Changed calculation of Import Propensities

Import propensities are calculated in a much more slick, Pandas style.
All columns which don't have all zeros get a 1 in the RoW row
Completed RoW creation
_relevant flows added to global_demo_model

Now only relevant flows (i.e. known country to known country) flows are
kept. Flows with no sector are not kept.
Added exception to country calculation for RoW
Moving responsibility for I and M to model