detailed user example vignettes

[cole-brokamp]

make into two separate vignettes, and link to from the readme

case crossover example

• simulate a data frame of case dates and geocodes
• apply Stephen's case-crossover code to generate matched control days
• use package to also include temperature and humidity

chronic exposures example

• simulate a data frame of patient IDs, geocodes, start and stop dates
• move example from readme on how to expand into a list of dates
• estimate pm25 and average it to monthly estimates
• simulate some model? and use average other variables for confounders?