if left unset, will return the user data dir as returned by `tools::R_user_dir` for this package.
nhdplusTools_data_dir(dir = NULL)
path of desired data directory
character path of data directory (silent when setting)
nhdplusTools_data_dir()
#> [1] "C:\\Users\\dblodgett\\AppData\\Roaming/R/data/R/nhdplusTools"
nhdplusTools_data_dir("demo")
nhdplusTools_data_dir(tools::R_user_dir("nhdplusTools"))