New chapter: example file-based data analysis project

[wlandau]

This chapter should

1. Show how drake can handle multiple file_out() files par target, and
2. Demonstrate dependency relationships where one target's file_out() is another target's file_in().

I would like to anchor on @tiernanmartin's code from ropensci/drake#257 (comment). The real work will be to grow this seed into a complete story, preferably structured like the gsp chapter. I think we should begin with a believable scientific problem we want to solve, explain the methods (I do not expect everyone to know spatial statistics or sf), proceed with the analysis, and see what we can conclude. We should iterate on the analysis to demonstrate what happens if we

1. Change a function dependency or a command for a target with an intermediate file_out().
2. Corrupt an intermediate file_out() and watch make() repair it.

[wlandau]

I no longer think this needs its own chapter, especially now that we have efficient formats.