Allow more complicated pool changes to accommodate multi-year variations

Namibia request. Notes from Luke: "It’s a more complicated kind of pool change and it brought up some other issues, too. They have agreements with other countries for in-migration. So Kenyan is sending nurses to Namibia for 2 year contracts. 200 nurses come in, 2 years later they leave (or maybe they all don’t? not sure exactly) and then 200 more replace them. They do have a cost associated for air fare in and out. In one way basically it’s just add 200 nurses one time that never leave until the in-migration program ends, but the costs are every 2 years. I’m not 100% sure that they get the same average salary as other workers, but maybe they do. They also have volunteer programs and I don’t think this supply should be averaging the salary cost the same way. But maybe there is some kind of housing allowance. Obviously we need more specifics on this. I think it can be worked out in the current system, but with more data entry so it gets a bit complicated. And there’s no way to account for salary variations."