Point locations for charter schools

[mscarey]

Currently, charter schools aren't being shown on the map because they don't have districts that can be shown as regions on the map. But they are reported separately on the map, and starting in 2017 there'll be a column in the TEA's data that says whether the row represents a district or a charter. We should either make a separate list of charters (with color-coded disparity data comparable to the map), or else add color-coded point data to a map for each charter school.

An idea for identifying charter schools prior to 2017 would be to take all the unique ID numbers in the TEA dataset, and then subtracting all of the ID numbers that match up with a district shape in the shapefile.