Move data files to 'data' subdirectory and change references to these files in lessons files

I created a 'data' subdirectory and moved all *.csv files into that directory. Then for lesson 01 - 04 I changed all references to the *.csv files to point to the new locations. New *.md files were knited from the *.Rmd files. New *.html files were created using 'make preview'. Additionally, I copied *.svg image files from 'bc' used in the construction of the *.html files for these lessons. Lessos 05, 07, and 08 do not require updates for files.
The difference here should be the search patterns. Please use the full argument name, `path` (here and below). I think that saving one keystroke is not worth the added confusion of explaining that R also accepts abbreviated argument names.