Improve data sets for demonstration purposes

[matt-dray]

• journeys_read.rda should contain data from more than one month; after all, it's meant to demonstrate what happens when files from two different months are read and combined
• separate CSVs of these two months should be added to data/ so that they can be fetched by the vignette when demonstrating the effect of oy_read() (i don't know whether this approach is legit)
• a non-Oyster CSV should also be added to demonstrate the warning you get from oy_read() when one one file is not in the right format