provide check that spatial aggregation doesn't induce spurious patterns

[sastoudt]

From this paper:

"one can induce geographic patterns in the aggregate data that do not exist in the input data"

Create a diagnostic to check for this (equations 2 and 3 in paper):

"The statistic S_j measures whether the region-level estimates for a given variable are within the margins of error of their constituent tracts. If a region-level estimate is within the margin of error of all its constituent tracts, then there is no information lost through aggregation; information loss increases as the 90 percent confidence intervals of more and more tract-level estimates do not overlap with the region’s estimate."

[sastoudt]

helper function here