add James Sibbet Demand Index

[DaveSkender]

[DaveSkender]

See related PR. I'm going to try to do some more research to see if we can triangulate onto the best formula. I suspect we may have to take Thomas Aspray's depiction, a second-hand interpretation of the original yet-to-be-found James Sibbet recipe, and compare it to what some of the more reputable charting sites are doing. In the end, we're just trying to find the best possible formula that will have the least amount of controversy.