add school districts in sf format

[dmi3kno]

This PR adds school district outlines and corrects school names in ames_schools for easy joining with the spatial counterpart. I renamed the dataset ames_schools to ames_schools_geo to avoid name collision and for consistency with other lat-long datasets. The sf dependency is suggested. I hope the error will be self-explanatory if someone will try to load ames_school_boundaries_sf without having sf package loaded. So long it is properly described in documentation it should be enough.

[dmi3kno]

If you don't feel comfortable introducing sf dependency, I have an alternative proposal. I can make spatial join to coordinates in ames_geo and record only Elementary School District name a particular property belongs to. Perhaps this could be enough, but would still add (hopefully significant) spatial feature. Let me know if you would like that instead, turn this down and I will prepare and PR a new column into ames_geo (I think this is where it belongs to).

Polygons are still sort of cool and can be used for AmesHousing2 package I am working on.

[topepo]

Sorry for the long day. This looks fine.