Save output as CSV, and read and write using `data.table`

Using RDS to read/write/store data is slow and not portable. We may want to switch the data pipeline or dashboard to being written in Python in the future, so using a format that Python can easily read would be preferable.

Looking at [a comparison of different formats and packages](https://waterdata.usgs.gov/blog/formats/), storing as a CSV and reading/writing using `data.table` via `fread` and `fwrite` seems pretty good. `feather` could also be an option (is supported in both R and Python) but it's not clear if standard dataframe and `dplyr` procedures work with it. `data.table` data can be seamlessly processed either with `dplyr` or with faster `data.table` syntax.