Rewrite data loads to use local data if available, download if not

[leehach]

Report code pulls data from Census file servers and API servers. Currently, data will be reloaded each time. Intermediate files (data frames, shapefiles) may be saved locally, and analyst may want to proceed using a previously generated intermediate file. These files should not be pushed to GitHub. /data directory has been added to gitignore. Code should try to load local files from data directory, and if not present obtain from original source or generate them from analysis.